Using 2 Wii U Game-pads causes Wii U games to run slower.
Earlier this morning at E3 2012 Nintendo made the announcement that the Wii U will support 2 Wii U game-pads, but after some digging it has been uncovered that the new features causes some problems. While the new controller has a many features like a gyroscope and built in camera, all of the processing is performed by the Wii U console. The Wii U game-pad is essentially just a shell, and only knows what to do when the Wii U console is turned on.
Nintendo has confirmed via Twitter that using two Wii U Game-Pads will cause frame rates on games to dip considerably, and that sacrifices had to be made to allow the Wii U to support more than 1 controller. Nintendo has promised that multiple Wii U game-pad games are in the works, but did not show anything at E3 2012.