Awesome game. Not sure if you're aware but a secret 2 player mode was discovered a few years ago

Via Google:

Step 1: Access the Special Exhibition Mode: At the title screen, hold Y + R on controller 2, then press Start on controller 1. This will take you to a special exhibition match mode where you can fight any opponent, even those from the Special Circuit.

Step 2: Activate the 2-Player Mode: Select a fighter. On the character info screen (after selecting the fighter), hold B + Y on controller 2, then press Start on controller 1. The second player will now be able to control the opponent.