Sharp has a 3D laptop out. It utilizes the left/right thing that can trick the eye into thinking its a 3D image. Maybe when Nintendo said it would be 2 screens, maybe its just 1 main screen that when "split up" displays in 3D. I mean, the Sharp 3D laptop has a 2D and 3D option, so it would make sense that if Nintendo was to go with what Sharp has out, a 2D/3D button would be possible.
Heres a link to what I was trying to explain
http://www.presencemultimedia.co.uk/weblog/entries/6