[hurin]

It's neat that they can play a duet that results in actual gameplay; but generally I think you are correct, a player trying only to win would inevitably play something pretty awful sounding. (It would be hard to establish a musical parallel to game play on one piano - let alone two!)

[smtucker]

Someone could do this in reverse, right?

Take the demo of a fight between skilled players and derive what keys they would have had to press on the piano to make it happen. I know that some (most modern fighters?) fighting games have options to show inputs on screen.

[hurin]

Sortof. For one if it could be done it would only work for that particular fight. Even that is highly unlikely though.

For one consider that music at least in terms how we (culturally, or biologically) are predisposed to listening to it: must keep time, you can change from 4/4 to 5/4 in the next section, but not arbitrarily (if you want it to sound like what people are inclined to call music at least).

(i.e. the drum-pad as a controller video someone linked in one of the responses, which is totally non-musical - is a good example).

Of course in a live-fighting game you don't care about structured timing, only about relative timing as to what your opponent is doing. Perhaps you could run a game through an emulator that enforced actions into particular time-blocks. Anyways I think it's very difficult, even to get something satisfactory rhythmically, not to mention tonality and a second piano.