Well we're trying to use the Gamepad it says we can't add a second player because its on the gamepad we even shut the screen off we can do it with two wii controls but we still can't find a way to do it :/
really digging the game so far though and ran into sincara last night too! XD
You cannot have the display showing on the Gamepad while trying to add a second account. If the gamepad is being used as your primary screen press down on the plus and minus sign then try again. When you hit the display button there is a prompt that shows up at the bottom of the screen letting you know that you can disable the display mode.