I purchased these last week and tested them out at a wedding. Wished I read this before hand! I had a second shooter with me and we were having some mysterious flash exposure issues. Turns out we were triggering each other's on-camera flashes. Hahaha. Anyways, we temporarily solved it by changing channels on the off-camera lighting and using one of each.
I finally got to test the "workaround" at home and while at first it was a little difficult to understand how things exactly worked, I managed to get it working.
2 x 5D3+YN-622+600EX
1 x YN-622 + 600EX on a light stand.
- YN-622 that is mounted on the 5D3 uses Group C.
- Via the 5D3's flash menu, set it to fire for A:B.
- YN-622 that is on the light stand is set to Group B.
So I am guessing that the YN-622 that is on the 5D3 is receiving on C and transmitting on A & B. Since nothing is "listening" for Group C, nothing is fired. Same with Group A, at least radio-wise. According to the document, the speedlight that is mounted on top of the YN-622 that is mounted on the 5D3 will be Group A. (?)
Something like that?