Last summer my friends Pit and Efrain sent me a link describing a strange experiment from the group Pop Levi using videos from YouTube for their single "Semi Babe".
You had to open both videos (each one of them featuring members of the group, one playing keyboards, the other one playing guitar and singing) in different browser windows and then resizing these to make both videos appear as one.
The tricky part is that each video were supplementary of the other one, so to speak, you had to synchronize them while playing to hear the song as recorded originally. A bit complicated but interesting storytelling though.
But later this evening i read (from Michael Lebowitz's Twitter, go check it out, it's full of cool stuff) about a site called YouTube Doubler, still in beta, that allows you to visualize two YouTube videos at the same time, just type the two URLs and let them go, enjoy!