Oh yeah, this was fun!
It's basically dueling pianos in the sense that you request the song that you are interested in to one of the two pianists along with a tip. If the tip is high enough, then he plays/sings your request (and the other guy joins in). If it isn't high enough, then your song gets put into a queue and it may get played later based on your tip. When we were there, a tip of $10 typically got your song played at some point, and a tip of $20 got your song played immediately. Don't request more than one song with your tip, or only one song will get played.
The performers are pretty awesome - they are good pianists and good singers and they have memorized all of the popular songs, old and new, as well as some more obscure songs. There's also another guy that raps, and a few times a night all of the waiters come up to the stage to perform.
Come early, because this place gets very very crowded after around 8 or 9 pm.