Nintendo actually used to make stuff like this. I had an add-on for my GameCube, for example, that let me play Game Boy Advance titles through the console on my TV. They were good times, but sadly, times that have been short since the release of the Wii, which failed to receive a similar add-on for DS games.
Enter the fantasy world of Dave Delisle, then, who has designed the DSU, a product idea that lets you plug a 3DS game into a small box (connected to a Wii U console) that would beam the top screen of a 3DS game to your TV and the bottom screen to the gamepad.
It might not be entirely practical for games that, say, scroll a single area over the two screens, but for those which show two entirely different screens (say, game on top and menu/map on bottom), it could work! I mean, you'd lose 3D capability, but for most games it's not like that matters.