This is pretty cute, too bad I'm terrible with rythm games and will never unlock the damn secret costume!
That said, with QWER it gets really, really cramped if one were to use two hands. I'd go with QW and OP.
To expand on what Sinoc said I also couldn't keep an eye on both the track and the stage and feel like a way to blend both in on the same screen could be more beneficial. I dunno, some transparency magic or some other shit, I'm sure it's doable.