I'd be pretty disorienting. Quite a few 3DS/DS games require you to be looking at both screens at the same time, and some require you to rotate the 3DS.
You know, I didn't even think about the gyroscope, but the gamepad does have one as well. They could have an option to have both screens on the TV for those types of games (you would still use the gamepad for the bottom screen's stylus controls/gyroscope). It might actually make the gyroscope games much easier to play, because you could just move the gamepad and look at the TV. It would be kind of tricky to do, but certainly far from impossible.