You would need two gameboys for two player. Then you would need either one or two cartridges to run it, depending on how I code it.
Some (commercial) gba games require 2 carts for two player other one. If they need only one cart then the master gba somehow has to send the game data to the slave gba, which is more complex. However If I can I would like to program this to use the single cart method, but I may initial go with the two cart method. Or I may not manage it at all!
In my program player 2 refers to the AI, although eventualy I hope it could also refer to a second gba.