Am I the only one who, whenever someone mentioned Pokémon XD, reads it as "ahaa!" in their head?
It's probably possible to have unbalanced double battles, although I suspect many scripts would need to be changed to allow for the missing Pokémon (knowing exactly what you've got to work with, 2 Pokémon per side, makes setting up the battles so much easier). It also seems rather unfair. It's not something I'd really look into unless I've got nothing else to do.
But it does give the player a challenge, but I suspect it is only going to be useful for the first 3 minutes of the game until someone catches a pokemon, unless they have the option to send out one pokemon.
Also I want to try to work on a tic-tac-toe game when I have the time.