The company is still researching whether the technology would allow it.
http://www.nintendoworldreport.com/news/26722
Shigeru Miyamoto says that Nintendo is looking into the possibility of using two of the new Wii U controllers simultaneously.
The initial idea was for one controller per system, and the company felt it would be more convenient for people to use their 3DS systems when a multiplayer game called for multiple screen controllers. However, Miyamoto said that Nintendo are in the process of: "Research about if someone brings their controller to their friends house and they want to play together on Wii U to whether or not something like that would be possible."
The controller could be $150, and the 3DS may have taken a drop by then - probably 18 months from now.