I was able to get both codes working.
In case it's not clear
To unlock the opponent selection screen, go to the title screen and hold R+Y on the player 2 controller and then press start on the player 1 controller.
After doing that, to unlock two player select your opponent and then hold Y+B on the player 2 controller during the opponent stat page, then press start on the player one controller. The player 2 controller will now control the opponent.