http://youtu.be/owBfU44NuGIDon't get too excited, though, it has lots of issues. I'll post details and instructions tomorrow. The video came out a bit dark, and got even darker after the upload, I hope it's not a big problem.
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Update:
So, here's the deal. I managed to make this hack work around a year ago, back when I was making some splitscreen gameplay videos. So I intended to make a decent, longer video, with friends and shit, but I got involved in other things and forgot about it. But I really had to share this with you guys.
The tutorial was made in October, 2013, but a few things have changed since then, so instead of carefully rewriting the whole thing, I'll just write comments in italics. Since it's too big to fit in a single post,
I'm putting it up in my Dropbox (notice that I use a free account, with daily bandwidth limits). I hereby declare it to the public domain, so you may use it as you please.
Edit: crap, looks like my video got reported as spam. Will look into it. Fixed (I hope). There, after having my video blocked again and being ignored by a dev, I just went back to youtube.