Same Character
Hold L + R on controller two at the player selection screen. Player two may now select the same character as player one.
Very Hard Difficulty
Wait until the "Press Start" text flashes four times at the title screen, then hold R + L and press Start. Select the "Very Hard" selection from the option screen.
Level Select
Enter the option screen and highlight the "Music Test" selection. Press Left or Right to select a level. Hold Select and press L, R(2), L, Start. Note: This also works for the Boss levels.
