From "The Other YN-622C User Guide" p.10:
Two-Shooter Setup (John UK technique)
A second shooter can share augmenting flashes. There is a problem – preventing the zooming and firing of the other camera’s on-top flash.
• Camera 1 set to E-TTL or Manual, and Firing Group A:B at some ratio/power.
• On-camera 622 set to Remote mode and Group C, with an on-top flash.
• Camera 2 set up the same.
• Enhancing flash stand with remote 622 set to Group B, plus flash.
• When either camera takes a shot, it's on-top flash is treated as Group A, the augmenting flash as Group B, and the other camera as Group C. Group C is not enabled in Firing Group A:B, so does not fire.https://docs.google.com/open?id=0B77OmmGIg0gMVFpqNkpBYXBHajA