YouTube Doubler is a new mash-up tool created by Brian Kane of Emergency Broadcast Network. It auto-plays any 2 videos off of YouTube side by side simultaneously.
For my first YouTube Doubler mash-up, I searched YouTube for William Burroughs and Aphex Twin, and these are the first 2 videos I clicked. Aphex Xtal video upload by wellofff. Burroughs video upload by whiteadderextras. Thanks to Create Digital Music for the heads up.