From my experience, I seem to have a solid team pair up with Sheik. Also I noticed Ness and Lucas seem to be good team mates for Link. Ness and Lucas are good because they're small so you won't have to worry as much about them with projectiles. Sheik has a great combo game paired up with Link if you space correctly. Also Link and Sheik have quick projectiles that are perfect for a quick stun if they see their partner in trouble. It takes 1 arrow or 1 needle to stun the enemy sandbagging your teammate which creates a perfect opening for your partner to regain control! Also, be patient and diligent. ESPECIALLY when it comes to 2v1(your team being the 2). This is where team work is crucial. Don't be overly aggressive. You could end up hitting your team mate which is obviously terrible. Also, be careful with the spin attack. It's a very useful tool, but use it wisely when on a team. You could accidentally semi-spike your team mate.
You have to get a feel for your partners' playstyle in order to work with them well. Once that is developed, focus on team work. You can single out the opponents 1v1, but if your team mate is struggling, make sure to give them a hand. Especially when it comes to recovery. You have an amazing recovery. Your partner should take priority. Not to mention you have a tether as well.
Most of the tips you can read on are pretty basic. It's experience you need my friend. That will give you the edge. You must hone your skills as a doubles player. You'll see improvement in singles from playing doubles due to it forcing you to be patient and know when to use what move in a plethora of new situations that you wouldn't encounter in singles play.