SSBM DOUBLES GUIDE

TEAM COMBOS

ALWAYS HIT/PASS TO YOUR TEAMMATE
You should almost always pass to your teammate unless your next punish move will KILL (either off the top or the side). This applies to GRABBING as well. Depending on your char/comp you should always throw to your teammate. There are a few exceptions to this:
Fox who should almost always up throw -> bair to pass to their teammate because his forward/back throws are not good.
Jigglypuff team combos should almost always go for grab into hold to guarantee a rest kill.
If your teammate is occupied by either doing their own combo, edge guarding or getting hit themselves then you should respond accordingly. There isn't a point into passing to your teammate when they can't follow-up.

IN GENERAL, you want to combo in such a way where the opponent is high enough % to be hit/passed horizontally with just a full hop attack. It's much easier to pass back and forth with alternating full hop attacks as it is much more difficult to combo back and forth at a short hop height because it gives way less time to position for the combo and less time to react.
Try to determine whose "TURN" it is for the next hit. This can be determined by the proximity of the enemy relative to you and your teammate. For example, if a teammate launches an enemy into the air directly above them and they able to continue the combo, then they should go for the follow-up. However, let's with that same launch example, instead of the opponent DI-ing straight up, they DI away and end up closer to the other teammate that didn't start the launch. Even if the person who started the combo is able to follow-up, they should let the teammate continue the combo as it is now in their space.

You also generally want to BACK UP when your opponent is about to pass.
Ex: you are next to your teammate and an opponent gets popped up and they are about to hit the enemy towards you. Often times I see the person trying to follow-up jump straight up which results in them being too close to react and missing. In this scenario, the correct thing to do is run AWAY from your teammate in the same direction the enemy is being passed to.
This will make it much easier to follow-up as the travel time of the opponent being passed gives you much more time to react to what your next move needs to be and where to position for it. Think about a football player going for a long pass. The same concept applies. They start running before the ball is even thrown. The higher % someone is the higher/faster/farther they will fly so you need to compensate for that space by running back even farther.

Remember to let the hit stun BREATHE
When you or your teammate land a launching move you don't want to both then follow-up at the same time. If that happens, then your combo ends immediately as you are now both stuck in lag from hitting the same person at the same time as opposed to the alternative which is alternating your attacks so that you can pass back and forth rhythmically.
There are a lot of frames of hit stun that can be used before passing back to allow your teammate to recover from their move's animation lag to be ready for another follow up after you pass back.

Try to always be looking at your teammate
This can be difficult to implement, but actively trying to practice this will make you 10x better at doubles. Always seeing what your teammate is doing will give you way more time get in position for a pass.

CHECK %s OF ENEMIES WHEN YOU GRAB THEM

If the enemy is in kill % HOLD THEM and wait for your teammate to come get the confirmed kill.
ex: sheik grabs someone at 90%, HOLD so fox can up smash. Team combos are great but do not back throw or down throw where you have to now tech chase or get a way less guaranteed kill follow-up. This is especially useful on floatier or heavier characters that can be hard to kill otherwise.
If they are not in kill % then throw to your teammate as normal or perform your own follow-up if your teammate isn't near you nor available.

TEAM ROLES

One person should generally be aggro and the other person should be supporting.
This is personal preference and also dependent on comp (ex: fox generally agro, floaty defensive)
The aggro player generally approaches or plays forward and makes openings for supports to follow-up with.
Support players bail the aggro player out of bad situations, save them, support their aggression with combos/kill/etc.
These roles are not always static. It can change constantly during a match based on many factors such as the positions that naturally fall into place or something like a player trying to focus on survivability due to being at a high % on last stock.

ALWAYS TRY TO SANDWICH

This is probably one of the most important sections on how to actually play teams neutral and what to look for positionally.
Do not have your aggro player fight their team while support sits behind, especially in 2v1 scenarios where an enemy is out of stocks.
If your support is simply sitting behind they can only bail the aggro player out of a bad situation if they lose the 1v1.
The aggro player should try to get BEHIND the person they are trying to 2v1 so the support player can approach and also pressure.
While the aggro player attempts to create the sandwich, the support player should move forward and take the remaining space and close in on the target. This way you can both attack the target at the same time and eliminate their escape routes (them dashing back).
In a more neutral 2v2 it isn't always ideal for someone to just run straight into the backline (especially vs slow double floaty teams), but it is something to to go for every now and then.
It's good to have this tool be utilized and practice it as well. However, just like any other neutral option, if you spam it, then it will become predictable.

SPACE AROUND YOUR TEAMMATE (NOT JUST THE ENEMY)

When you are 2v1 pressuring a target in a sandwiched position (enemy between you and your teammate), space your moves such that they will not OVERLAP to the other side of the enemy and intrude on your teammate's zone.
If you and your opponent or on opposite sides of an enemy then you should both be pressuring the enemy between you two while remaining on your respective sides. You don't want to end up entering each others space while pressuring/walling as that could result in you hitting one another. If you both remain in your zones then both players can pressure relentlessly.
This applies vertically too. If an enemy is on the platform and you and our teammate are both on opposite sides of the opponent you can still apply this concept to this scenario. Often times I will see someone throw themselves on the platform while someone else tries to poke at the same target and they end up hitting each other. You must space around your teammate as well by establishing these boundaries and being on the same page with pressure rules.

PRESSURING

When pressuring with hit boxes, you should alternate move timings to keep people locked down. This can generally force an opening (shield poke, roll, jump, panic option, etc.). You generally don't want to both throw out a move at the same time as this effectively means you are both covering the same option at the cost of both of your players. If you both attack a shield at the same time and the opponent does something like roll you are now both in lag and the enemy escapes. However, if one person instead waits for the roll while their teammate shield pressures you are now covering multiple options which is much more difficult for enemies to deal with.
You can also setup for grabs this way while your teammate is pressuring someone's shield with hitboxes.
Pressuring this way restricts their options to generally hold shield until they are poked, or to roll which can easily be covered on reaction with alternating pressure timings from both teammates.
Dedicating someone to grab and the other person to hitbox pressuring can also be good as you instead cover 2 options guaranteed. Just be aware that roll can beat this, but it is much harder for the opponents to guess correctly when you are covering multiple options at once instead of both teammates only covering 1 option (throwing a hit box at the same time) that loses to shielding.

DO NOT PRIORITIZE EDGE GUARDS (INSTEAD PRIORITIZE FORCING 2v1 SCENARIOS)

GO FOR THE 2v1 after hitting someone off stage (especially FLOATIES that are NOT EDGE GUARDABLE). I see so many people try to edge guard puff, peach, samus. These recoveries not only take a very long time, but these characters are almost always going to make it back to the stage regardless if someone is edge guarding them or not. Instead, you can go for a 2v1 or help your teammate which can net you WAY MORE. These characters are NOT worth edge guarding EVER in teams unless it's a final last stock 2v1.
Even trying to edge guard spacies when they have several recovery option mixups like double jump, side b, multiple up B angles, etc., you are better off going for a 2v1 attempt.
A 2v1 from an experienced team is way more valuable than an edge guard attempt as an opening can lead to BIG damage especially on fast fallers. Also, having an opponent at high % isn't bad because they are going to get put into a situation where they have to recover more. That gives more opportunities and time to fish for a 2v1 scenario/combo.

Another reason edge guarding is generally worse in teams is because if the enemy team is experienced in doubles, they can make it quite difficult for someone to edge guard them. They can do things like stall their recoveries and wait for their teammate to come save them. There are also things like falco lasers and sheik needles that can help interrupt edge guards or save their teammates. These are all the more reason to continue the 2v1 on stage and not force an edge guard.
"BUT MAH GIMPS" - cheeser ass sheik and marth players doing back throw/dthrow... Think about how long it takes for marth to dthrow dtilt a fox and then go off with either fair or reverse up B or some other follow-up to finish that edge guard. Now think about how much LESS time it takes for an enemy fox to run over and shine. If someone goes for an extended edge guard it is very likely it will be interrupted. GO FOR THE 2V1 INSTEAD!
Edge guards CAN sometimes be worth going under the right circumstances. I think it's ok to edge guard when:
The enemy has very limited recovery options that can guaranteed/easily be covered by you alone (they can only go to ledge)
The enemy teammate that is on stage is occupied (getting comboed by your teammate, also recovering, dead, etc)
Your teammate is in a position where they can/will wall off the enemy from trying to interrupt the edge guard
With all of that being said, if it is a 2v1 scenario where there are only three players on the screen you should always edge guard with your teammate.

PERFORMING A 2v1 EDGE GUARD

When edge guarding in TRUE 2v1 scenarios you should almost always have one person cover ledge by grabbing it and rolling while the other person cover the on stage options. This way, if a player being edge guarded goes to the ledge, they will die from your teammate edge hogging them, but if they choose to go on stage then the other player is there to cover the rest.
A general teams rule as to who grabs the ledge is usually whoever ends up being closer to the ledge grabs it, However (if there is time to choose who grabs the ledge), I would also recommend assigning the character that is worse at covering the on stage options to grab the ledge so that if the enemy does not choose to go to ledge there is a higher chance the on stage character can successfully finish the edge guard.
Often times I will see someone on the ledge do something like ledge hop bair which is BAD because this allows a mix-up option for the opponent recovering to go to ledge safely. An even worse outcome of that same scenario is the teammate on stage then tries to cover the on stage option and now both players trying to do the same edge guard hit each other. Don't do this! Dedicate one player to the ledge and the other to the on stage options.

COMMUNICATION

TALKING can be necessary at times during the game and is definitely necessary outside of the game.
Little in game call outs can be very helpful. Stuff like:
I need help (when recovering)
I need ledge (when recovering)
I have ledge (when edge guarding)
No jump (when an opponent has no jump off stage)
etc.
More information you can give to your teammate can never hurt.

Outside of the game you should be talking strategy. This can include things like:
Telling your teammate to create more sandwich opportunities.
Pointing out enemy habits you notice to your teammate (ex: at the start of every game their fox player jumps to top platform)
Discussing roles about who is going to grab during 2v1 pressure (usually comp based for example sheik should generally grab).
(Tell them to read this guide xDDDD!)
etc.
It is important you discuss whatever that may be with your teammate. Even if you differentiate strategies from this guide it is WAY more important to be on the same page, and you will only achieve that synchronization through discussion.

GENERAL BONUS TIPS

Light shield spam is very good at dealing with getting 2v1ed especially vs less experienced doubles players that just spam hitboxes brainlessly and don't grab/pressure correctly.
While you wait for your teammate to respawn, jumping to platforms and light shielding or going to ledge for intangibility are great ways to safely stall out the time it takes for your teammate to come back. This also means generally you want to avoid fighting 1v2 as much as possible (as the one, not the two) as it's generally a disadvantaged scenario.

ALWAYS try to recover even if you are "dead" or don't think you can make it back. If you are teaming with someone that has good saving tools (falco with lasers, sheik with needles, peach/puff being able to fly, etc.) and your teammate has good awareness, there is a high chance you can survive.
Almost ALWAYS try to recover high especially if you are a fast faller player.
You also generally want to up B almost always vs side B. If you up B and go high this gives your teammate so much more time from them to come save you compared to the short amount of time side-b gives them. Recovering high also allows you to live way longer and will require a much longer time for the enemy team to edge guard you which will again give your teammate more time to save you or interrupt their edge guard.
Also shine stalling and stalling with zelda/sheik down B are useful tools to stall your recovery to buy time for your teammate to come help you.

Going for RISKY saves (like fox jumping out with up b) are generally NOT worth it especially if you are low % and especially vs characters that can edge guard or gimp fox easily (everyone in the game lol).
When you are last stock high% it is more important to focus on staying ALIVE rather than forcing aggro sandwich scenarios. This is especially true in last stock 2v1 scenarios. A team that can properly 2v1 should almost never lose to the solo player if all %s are even and skill levels are roughly the same.
Play support and be there to bail our teammate out if they lose 1v1 interactions. You are also valuable alive to perform TEAM edge guards which are way more guaranteed than a 1v1 edge guard.

Try to always be looking at your teammate. This is SO important and is a huge part of improving teams awareness. It might be awkward at first but it is very valuable to practice looking at your teammate more even if it means not playing your own neutral as well due to focusing your attention elsewhere. Looking at your teammate gives you the most information possible as quickly as possible. When you look at your teammate you will have more awareness of the situation which will give you more time to get in position to follow-up on their pass or know if they need to be bailed out of a combo.

WRAP UP

TRY TO FIND A STATIC TEAMMATE! Practicing all of these things with a STATIC teammate will help your improvement tremendously. It can be hard to practice these things in pick-up games. From my experience most melee players, regardless of how good they are at singles, tend to lack a lot of basic doubles fundamentals which makes practicing these things very difficult without a consistent teammate. Playing regularly with the same group of people can also help even if you are switching up the teams.

Most importantly, have fun :). Doubles is super sick, underdeveloped, and unexplored. I truly believe it is a very underrated way of playing the game. I think people are often frustrated or turned away from the format because they don't know how to properly play and it can feel janky without knowing some of the fundamental teams which is understandable because there is so much more happening on the screen compared to singles. Part of why I wrote this guide was to fix that issue and hope that the people interested in doubles have some direction to work with so they don't get as frustrated or feel as lost when learning the game and end up writing of the 2v2 format completely.