So I searched through the forums to see if there was already a topic about 2 vs 2 battles (Duo battles) for either pve or pvp, and I have yet to find anything on it. So I decided to put my thoughts out there, even if I am a noob to this game.
Duo (double) battles or 2 vs 2 features up to 4 pokemon in battle... There are several concepts that you can create for both the PVE players and the PVP players. For a PVE option, since this is a MMO version of pokemon its nice to have sort of a Co-op feel to the gameplay. A great deal of us out there joined the game with some friends or maybe you found someone while playing single player and you guys wanted to do some co-op style pokemon battles. What better way to get that feel than to have NPCS for double battling? Having a battle tower, where you and a friend can team up to take on random NPCS could be an amazing addition to the game. You could even have a singleplayer option for the lone wolves out there!
For the PVP option you could make a Duo Ranked Tree, so that two friends could team up!
I have always liked double battling in the pokemon games, and it would be pretty awesome to have that in this game. And I am really curious to see what other people have to say about this.