Details: Like all previous Battle Facilities from past generations, The Battle Tree, too, includes the option of Multi Battling. Here, a player can participate in Double Battles, but with two trainers on a team against two other trainers, each selecting only two Pokemon. When one begins the process, they are first asked if they wish to play with a friend via local connections through 3DS Wireless Connection. (Your streak will reset if you pick this option.)
If you don't have a friend locally that you can play with, you can play with an AI Player (Computer NPC). But with The Battle Tree, there is a new feature that can be used with the AI Players you can possibly team with. As you go through any battling mode apart of the Battle Tree, a player can spend 10 Battle Points to Scout the given trainer. By doing this, this will allow for them to join you as a potential partner in Multi Battle Mode.
This can even be done with Special Trainers like Red and Cynthia, and other returning characters for older games. You begin with only one trainer to team with, Pokemon Breeder Rada; however, you can add Lillie to this list if you speak to her in Professor Kukui's Lab. And the Pokemon the trainer brings in Multi Battles will always be the first two in their Pokemon Parties when you battle them in other modes.