Press the play button to play both videos side by side. Use the adjuster to change volumes between the versions. Probably works best in Chrome, but your miles may vary.
The volumes have been normalised and the videos don't start at exactly the same time which I've tried to adjust for. If the video is not quite in sync, try stopping and starting a few times (which usually works, if not - reload the page). If really needed, you can adjust the start delay with a millisecond value (1000 = 1 second) using the box below. It depends on how fast YouTube loads the videos, but 400 or 550 seems to work very well.
Viewing on an iPad or iPhone? Sorry - multiple simultaneous video streams is not supported 😔
Preparing videos!