Contra 4 uses the gap as gameplay space, indeed. I just tested it. Also, once I played an Arkanoid game that used it. Sonic doesn't use it, I perfectly remember because it was so annoying. But Contra is difficult too. I don't see a way on which playing in 2 screens can be comfortable.
About the console, I don't see the point. Yes, many parents with little kids will buy this for sure, so from a marketing point I understand, but there's plenty of us, kid adults who play.
I want a 3DS with BIG screen.