Both Be the Same Player
This trick lets you both choose the same character in a 2p vs. battle. Start a one-player game and get defeated. When the Continue screen comes up, press Start on Controller 2. Now, pick the character that the first person was playing with, and press Start again. As the game begins, press Start on Controller 1 to play the game with the same person.
Start a one-player game. When you run out of continues, before your last man dies, press START on controller 2. You can keep playing as Player Two with a full set of continues.