You need an integer variable created on both player/controller and one on the door BP. Make the door variable instance editable. On overlap of the box trigger set the player integer to the integer of the door that particular trigger is in front of. Then make a function in the player that gets all door actors, run a for each loop and check player integer vs door integer. When they match you have the door integer you want to possess. So when you use the possess node and it asks for a controller number you use that integer. Then you will possess the correct door.