That awkward moment when your logic that should be right is wrong.
Do you seriously not see the line of the frame between the two screens? Where on earth do you get that the top and bottom portions are all one single screen?
That doesn't mean anything actually. It actually IS a single screen. And a touchscreen at that. The upper part is protected by a shell of plastic that prevents the touchscreen from activating.
http://www.nintendolife.com/news/2013/08/nintendo_2ds_is_actually_made_with_just_one_screen