Make quick side-by-side video mash-ups using YouTube Doubler. Be warned: as with many sites featuring user-generated content, this site has a lot of inappropriate and immature mash-ups in its "Top" and "New" lists.

The site has a simple interface where you enter the links of two videos.  You also have the option to include a starting timecode marker for each video. Anyone who opens the link (generated in the address bar) will be taken to page with the two videos playing simultaneously side-by-side at the starting points you entered.

If you want to avoid sending people to the actual site to view a mash-up, you can embed the mash-up as I did above using the html code. Unfortunately you have to start the videos on your own, and they do not loop automatically as they do in the YouTube Doubler site (see here). 

Note: I also did a find and replace in the html code on the video dimensions in a text editor program (Notepad) to make the embedded videos fit within my website layout.