How can I run two Arduinos in sync for video generation?
I would take a look at MREtv which seems to promise all that you want to do, with just one arduino. This might not be the answer you're looking for, but getting threads to sync is a pain in the rear end, see this for explanation of why threads are a pain.

Now back on topic, it seems like MRETV might off what you want to do, and it can do some pretty impressive things from what I read in the arduino. You might need to modify the library a bit, but it offers already implemented encoding, so that is a plus.

http://www.instructables.com/id/MRETV-Video-Stereo-Sound-and-much-more-from-a-s/