Hmmmmm... a summary of Peach in doubles.... Alright... this might be a bit long, but here I go
Overview:
Peach has to be the supporting member of the team. Even though Peach has nice combos that an rack up damage quickly, you can't expect to rush in there and smash your opponents around until they die. Playing Peach requires thinking and spacing. You can't die, and you can't take damage. That's how you play Peach in doubles
Stock tanking:
Stock tanking is extremely important. Peach has to let her partner take over, while she stays back and stock tanks. Because you're partner is doing the work, they might die more than they should. If they die, and you haven't been stocktanking, then you're left all alone with one life left to fight the 2v1. If you have been doing a terrible job, then you'll die even before your partner. If you have been stocktanking, then when your partner is out, they can proceed to steal your life and resume the game. It's important to stock tank, because it makes the end game a lot easier.
Damaging:
It's important to do as much damage as you can while taking as little damage as you can. A good way to think is that for every % you take, you have to do twice that % to your opponent. If your partner is good at killing (Snake, D3, DK, Zelda, etc) then it isn't the end of the world if you use a couple of fairs or fsmashes for damaging. If your partner isn't too great at killing, however, then it's a good idea to save your killing moves. Comboing is your best way to damage your opponent, with dairs, uairs, nairs, etc. All your Singles tips can apply (jab, grab, etc). Turnips are also great, even if you often don't have the time to pull them out. Racking up damage is Peach's main job. A good thing is that doubles can be chaotic, so your moves might be easier to make contact with at least one of your opponents.
Killing:
Killing usually shouldn't be left for Peach, because partners often have better killing moves than her. Snake has utilt, D3 has utilt, MK has dsmash, etc. Peach usually shouldn't have the role of killing, but if either your partner is having trouble killing or if you just have to kill, then it is possible. Doubles can be chaotic, which means landing that killing move can be a little bit easier than in singles. I know I get a lot more usmashes in doubles than in singles. Fair, fsmash, usmash, nair, utilt, uair, bair, etc are all good killing moves. Just make sure you don't hit your partner :O
Support:
Playing the role of support requires....support. In doubles, you have to be watching all 4 people in the match. If your partner is ever in trouble, then you have to save them. If theyre getting CGed, then do whatever you can to get them out. If thy're getting comboed, then help them out. Always be wary of where your partner is, how they're doing, how much damage they have, how much damage their opponent has, etc. If their opponent has high percent, but your partner is having trouble killing them, then get in there and kill their opponent!
1v2:
If you're ever in a 1v2 (you're by yourself and both of your opponents are still alive), then you're in for a fight. I'm not too amazing at these, because I just end up using a ton of fairs and fsmashes to space, wasting my killing moves. Your best option is to focus on one opponent until they die. Take out the one who has higher percent. If they're around the same percent, then take out whoever would be more difficult in singles. Then, when they're dead, you can play the rest like singles.
2v1:
This can be really easy or really hard depending on your teamwork with your partner. The easiest way to approach this is for one of you to get a grab, and then infinite grab release until they're at killing percent. If not, then you can try comboing them with your partner with dairs, bairs, and nairs. If not, then you can just have Peach do as much damage as she can until she dies, then let her partner do the rest. It's a desparate option, but it works
Conclusion:
Peach can be amazing in teams if you play her right. I know that this quide seems like you have to be campy to win, which is true for me, but it varies among every player. In my opinion, supporting is they key to destroying in teams