The recipe for a good multi partner is a trainer who brings 4-attacks sets, since you don't want to have to count on your teammate displaying proper discretion with how to use their status moves. Even better if their lead uses a choice item, since once you get past the first turn you can be absolutely certain what move they're going to pick next time, and can plan around it. Feel free to bring your own support moves to build around the sets they have, just don't try to expect them to show the same courtesy to you.
Some favorite sets of multi players:
Choice Scarf Entei/Typhlosion (these almost always spam Eruption, hitting both opponents at once so you don't have to guess who they're going to target)
Choice Scarf Garchomp (this almost always spams Earthquake, especially if your lead is immune to ground which it really should be if you're using this as a partner)
Mega Lucario (has Bullet Punch, Close Combat, Blaze Kick, and Rock Slide for pretty diverse coverage)
Mega Metagross (has Bullet Punch, Meteor Mash, Zen Headbutt, and Brick Break--it can get bitten by accuracy problems, but if not you can watch it hit ridiculously hard)
King's Rock Greninja (Water Shuriken, Grass Knot, Rock Slide, U-turn), especially if you get one with the 1/3 chance of having Protean
