I imagined this was for events like weddings, gatherings, one-room meetings. I -- personally -- would have wanted a manual flash lighting the room attached high (best) or on a light stand placed inconspicuously. (I keep a weird set of clamps and hardware for this.) I would not want that level to change, although of course throughout the space its level will be different. Then I would try to have my on-camera strobe act as fill to a light that appeared to be ambient but was actually from that "room-bounce" or "room-direct" unit. Having two people able to use that wrap-around light in a neat idea and I hope you can make it work.
I have been avoiding ETTL. Pure fear, lack of experience, I admit! So I cannot tell you if ETTL is retained with one of these splitters.
So this setup is two senders and two receivers. I use Odin, still learning all its capabilities. I think if I were to do this it would require a separate Odin controller, and I never asked what just a controller costs.
Hope you can find a good solution for your particular needs.
Thanks Jonathan. Yes this is mainly for weddings. The issue obviously is that a chapel can be quite large so unless you can position the "ambient" light far enough away the falloff can be pretty quick and if you can put it far away you're giving up light output and softness in exchange. I have done something similar to what your describing. Instead of Speedlights I used a Alienbee bounced off the projector screen. It worked pretty well. Only think I might have done different is using two instead of one so I could have one on each side. If you're interested the ceremony shots are were shot this way here:http://leightyphotography.com/weddings/crystal-and-jeremy
The main issue I had here was that since it was manual I didn't have ambient light output adjusted automatically and was close to having quite a few of the shots blown out. Hence my interest in checking out doing the same sort of thing but with speedlights.