I found a website that helps you create youtube video mashups by playing them side-by-side simultaneously. It’s a called Youtube Doubler.

This is a mashup I made of someone beating the high score on Game Boy Tetris (every Game Boy came with Tetris) and ABC’s coverage of the Berlin Wall coming down (It turns out that Game Boy came out the same year that the Berlin Wall came down, 1989). Click the image to watch the mashup.