In two players mode, lose all your energy with one character so you are forced to use a continue. Now, lose all your energy with both characters and when you are prompted to continue the game, press Start on both controllers at the same time. If done right, instead of one character using the last continue, both characters will continue and the counter will read 99.
Debug Menu
Pause the game, hold B + X, and press R to open the sound test menu.