There has been some going back and forth on whether the Nintendo Wii U would support two tablet controllers at once. We’ve heard from developers a while ago that the Wii U controller takes up considerable processing power and therefore two of them would be unlikely. Then Nintendo said that theoretically, the Wii U could support two controllers, but that they company is only planning on supporting one.
New rumors (via NeoGAF) now suggest that the latest version of the Wii U dev kit includes dual controller ports for the Wii U tablet controller. While the retail version of the controller will be wireless, the dev kits have wired versions. According to the rumor, the second controller port isn’t usable by developers, but its presence suggests that at some point in the future, a second Wii U controller might be possible. Since the controllers do take up some system resources, developers would have to keep this in mind when creating games that support dual controllers. Another thing to consider is that the tablet controller is much more expensive and advanced than regular video game controllers. Nintendo currently sells the Wiimote at around $30, the Wii U controller could easily cost twice that, perhaps even more.
When it was showcased at E3, the Wii U was playable with one Wii U controller and up to 4 other Wiimotes at the same time. Nintendo revealed mini games where up to 5 players (one tablet + 4 Wiimotes) could play at the same time, either together or in splitscreen. A recent leak showed that a new version of the controller with dual analog sticks, a larger size, and a slightly larger screen as well. You can find a comparison between the old and the new controller here. If dual tablet controllers are possible, we expect Nintendo to announce it at E3, perhaps with other Wii U hardware announcements.