Amazon has listed some limitations in regards to the upcoming Hori Pokken Tournament Pro Pad Limited Edition Controller, which you can preorder from the shopping website with this link. One of the restraints is that the gamepad can only be used with the Pokken Tournament as it won’t function correctly with other games on the Wii U.
The other restriction is that you can only use one of the Pro Pads as the first player is required to always use the Wii U GamePad in Pokken Tournament for the local versus mode. This is probably due to the camera’s focus on one player’s back with the other player appearing in a distance. As a result, the developer at Bandai Namco most likely decided to use the Wii U GamePad as a screen for Player One and the T.V. for the other screen for Player Two.
Pokken Tournament is expected to come out early next year for the Nintendo Wii U. Physical copies of the game comes with a Shadow Mewtwo Amiibo card that will let you immediately unlock the special character. If you are planning to play a lot of multiplayer locally, you might want to check out the above-mentioned Hori Pokken Tournament Pro Pad Limited Edition Controller, which currently has a listed release date set for March 18th, as well.