Scott sent a”Multiplayer Demo”to gbadev.org. Here are the details:

ere’s a little demo that shows how to replicate a MULTIBOOT ROM across a link cable (without a 2nd flash cart), and then exchange structured data between the gameboys. Once the ROM is replicated, the gameboys call a function called ExchangeStructs() once per game loop. Each player has their own STRUCT that you can change to be anything you like. The function handles the multiplayer transfer, ensuring that everyone has a”current state”view of the game.

http://www.gbadev.org/