I'm no expert either, but with someone who has a lot of doubles matches under his belt (friendlies mind you) I can give some general tips that I feel a lot of people don't actually know..
1. Try and pick the weakest link mid-match and zone him out, seperate him from his teammate and do as much damage as possible. Do this consistently and you'll be surprised how many games you'll come out in the lead from. Eventually you and your bud can do this almost psychically.
2. Never interrupt your teammates combos, merely add onto the end of them. Sure when you get really good and you trust your teammate you can intertwine a combo but that is some serious sh**.
3. In competitive play, never be afraid to take your teammates stock. Make it a habit to say "No, I need that stock", and only say that when you're absolutely sure you can 1v1 or 1v2.
4. When edgeguarding make sure you cover the options your teammate has not, if he's on the ledge be prepared to use that kill move on a recovering opponent, if he goes high try and get the ledge covered. Against a tether try and grab the ledge so he has to jump up and your teammate can land that free smash.
It's funny for that number 4 recently I had a game where I wasn't doing so hot, my training partner yelled Forward smash and I threw out a forward smash (With Zss). Ending up getting the kill on the recovering lucas. Communication is key in doubles, some people do it without talking others do it with keywords, Whatever works for you.
