How do you disable onboard clock or is it automatic?

You'd likely have to de-solder the one on the board and maybe do a few other small modifications to get it to work properly. I imagine you'd want to use a completely different kind of source and then use some op-amps to both shift the waveform and also then strengthen the signal so that the load of each camera doesn't cause issues.

no analog at those frequencies, you would want dedicated Multi-phase Clock Buffer chip, something like a couple CY7B9945V

I think you could probably just parallel connect the quartz clock terminals on the cameras.

As long as your leads didn't have too much capacitance, it ought to just work.

The phase shifting you can do in software by (for example) restarting the camera hundreds of times till by chance it starts a frame with the phase offset you want, and then just run on from there.

Won't two clocks interfere with each other?

As long as they're tuned close-ish to eachother, they will synchronize and oscillate in lockstep.

I'm talking the actual quartz crystal here, not the output of some digital clocking chip (which you're right, wouldn't work).

its already external, on the back of each camera

But it's only selectively enabled no?