(their logo I really don't know what it's supposed to mean)YouTube Doubler is a site that allows you to combine 2 videos to play at once, without you having to by switching tabs and such. Start time is configurable, and you can share the combinations to others. I find it fun to get the source of a YTPMV/remix and combine it with the actual YTPMV/remix. It's weird, but cool.
It's not PERFECT, so it won't be in sync if you're looking for perfect sync.
Here's one I did,
nichijou op2 vs. Nichijoint - My Ordinary Weed. You may have to refresh it a couple times to make it sound like they're actually combined. Or don't.
