How can I synchronize the framerate of two webcameras. Is it even possible?
I'm afraid you're almost certainly out of luck. Your "naïve idea" isn't bad as a starting point, but the details are killers. First, the crystal is just part of an oscillator circuit which is contained in the SunPlus ASIC. It's not certain that the resulting clock signal actually appears outside the ASIC, although it is probably sent to the camera. If you can find that clock, in theory you can make a buffer circuit which would send a clock to the slave board. Problem is, you probably can't just feed the received signal into the ASIC crystal connections. (Maybe you can, maybe you can't, it depends on the circuit.) But if you can, this only locks the pixel clocks on the two boards, and does not guarantee that the frames are synchronized.

At a guess, each board has an RC combination which is used by the ASIC to produce a POR (power on reset) signal, and if you can find that you might be able to generate a signal which is sent to the slave board, and both boards would start in the same state at more or less the same time. 

This would, in theory, do what you want. I say in theory. There is no way of determining at this remove what the driver software in the receiving PC does to the ASIC internals, and you would have to get the driver to configure both cameras more or less simultaneously. And I have no idea how you would do that. You can download the drivers from Logitech and try to reverse-engineer the code. Lots of luck. Neither Logitech nor Sunplus are likely to release the ASIC details, either.

So, I really can't recommend this as a starter project.