At the title screen, press Select, Left, Right, B, B, Down, Up, Select, B, Up. A sound will confirm correct code entry; a fourth menu option will also appear, corresponding to the mode.
Super Game Boy 2P Battle Mode
Note that this cheat requires the use of a Super Famicom and a Super Game Boy peripheral. Enter the Message Test cheat as described above, then go to the Options menu and return to the title screen. Enter Select three times, and then another sound will play and a fifth menu option will appear, corresponding to the new mode.