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While not confirmed, the Wii U might end up supporting two controllers at launch. Previously, around the E3 2011 timeframe, Nintendo was non-committal about featuring more than one controller for Wii U. In the past, technical limitations may have been thought to restrict the amount of controllers the console was capable of supporting. But Develop now claims that Nintendo has since modified the system's hardware so that two Wii U controllers can be used simultaneously.
Develop’s source said:
“Nintendo now know they absolutely need to support two tablets. At E3 they didn’t commit to this, but they know how important it is to make it technically feasible to support two screens. Even if that affects framerate, as a developer and player, I don’t care. It needs to work. Developers will design appropriate games for this. If you’re building a quiz game you’re not going to give a shit about the framerate.”
If only two Wii U controllers is all we'll be seeing, then that's fine. The remainder input devices can just be used by the Wii remotes.