Page "Game Boy Player" Paragraph 4

All controllers, Game Boy Advances, and Game Boy Advance SPs connected to the GameCube are recognized as the same player.

This allows a sort of co-op mode for games that don't normally have it ( most likely this was not intended by Nintendo ).

Furthermore, allowing for multiple controllers recognized as the same player allows for simpler and more comfortable play of single system multiplayer Game Boy Advance games, such as those found in Mario Party Advance.

This is in lieu of up to four players holding one Game Boy Advance unit.