Supposedly processor speed and RAM still in flux as well.

Nintendo’s engineers are on the cusp of working out a way to make two Wii U controllers work with the system, according to an Develop’s “inside source”. This represents more of a challenge than traditional controllers, as the system needs to be able to stream both video and audio to the iPad-like touch screens. Given the difficulty of implementing this second touch pad, Develop’s source ruled out any future possibility of the system supporting the traditional four controllers, without hooking up your classic controllers and Wiimotes.

Initially, Iwata said that he didn’t want to add two controller support, as the cost of two touch screens would be prohibitive to casuals. Interestingly, the source did speak to the possible performance compromises the company might make to support two simultaneous controllers.

“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 **** about the framerate.”

While this is potentially troubling news for people hoping for multi-controller 3D games, he or she also said that both the systems RAM allotment and processor clock speed had not been finalised yet.

Source: Develop Online