This works on the AES and CD versions. Input this code on the options screen.
On the character select screen, hightlight these characters in this order (but leave the highlight on each character for a couple seconds before moving to the next): Billy, Marion, Changfu, then Jimmy.
At the character selection screen highlight Billy, then Marian, then Chung Fu, then Jimmy for 3 seconds each. You may now choose Duku or Shorko to play as.