During the Nintendo E3 Media Briefing, it was casually mentioned that the Wii U console will be able to support up to two GamePad accessories at the same time. It's nice to know that some games will be able to allow two players to use the more advanced features that the GamePad has to offer, but there is one catch: when using two GamePads, the frame rate of the Wii U will drop to 30 frames per second. It's not horrible, but it is a performance decrease, so it should be noted. You'll also be able to turn off the second GamePad and get that performance back on the fly.
Check out the video tour of the Wii U GamePad for more on the peripheral.