Synchronizing two OV7670 image sensors
I very much doubt you will be able to achieve synchronicity by issuing I2C commands, but it is worth a try. Check with a scope how I2C commands affect the frames.

I think you could just connect the I2C pins of both cameras in parallel.(Assuming you don't need to read from the camera's, IIRc that is not needed unless you want to do something fancy.)

I it fails, you could consider clocking one camera chip from a PLL that is driven by the phase difference of the two frame (and maybe line) pulses.