01-12-2015 04:03 PM
Using a 600ex-RT in optical almost seems like sacrilege.
Another option would be to get a set of Yongnuo RF-602 triggers (about $20 for a set), and use those to trigger your 600ex-RT. You could then control power of your 600-ex-RT in hand, but it'd be manual only.
01-12-2015 04:23 PM
You could also use the Yongnuo RF-622, which would allow eTTL shooting. It also has a (eTTL capable) pass through so you could put another 600ex-RT, or your ST3, on the on-camera trigger to do it's own thing. Though at that point you might as well just get another 622 for your other flash. But putting Yongnuo triggers on a 600ex-RT also seems like sacrilege.
If it's just an occasional thing, then work with what you have. But if it's something you're doing routinely, I'd recommend not using workarounds.