Select Gameboy Advance games had the ability to play multiplayer games using only one game pak (cartridge). Also referred to as the “multi-boot” function, these single pak linkable games often played like a demo of the main game. However, some had full blown multiplayer options that were quite fun.
This video demonstrations how to link up multiple GBA systems using link cables and only one cartridge. Keep in mind, Gameboy MICRO systems required a special adapter/cable to play multiplayer games of any kind.