At the options menu, hold down Select and press Up, Down, Left, Right, Down, Up. Then the "Players 1 or 2" option will be unlocked, which allows the two player mode to be played on two Game Boy Color systems using a link cable.
Select Ralph as a character, then intentionally die. Then, quickly press Select, A, B(2), Up, Down, Forward, Back after transforming into a person.
Note: You must have a 2.1 or higher version of the Game Shark to use these codes in a Game Boy Color system.