for a:b both are metered seperately to find out how much power each has to provide to get the desired lighting ratio.
Interesting, in that case it's even more strange Canon doesn't allow three independent a-b-c groups with independent fec for the optical legacy system if they do 3 preflashes anyway...
You can select HSS and ETTL in pre 2012 bodies, it just is nowhere near as consistent as non HSS ETTL, I get regular +/- 2/3 exposure variation across HSS + ETTL use where as for same framed images ETTL by itself is very consistent.
That's very valuable information, I might be experiencing the same thing with the 60d - though it's hard to pin down where the problem is, but I'll keep watching for it if I do optical (I still have a 430ex2) vs. rt.
I am just very glad they left us the ability to actually select our standard sync speed with the RT system on the pre 2012 bodies.
Yes, somehow doesn't sound like them :-p
There just might be some weirdness gremlins be lurking in there.
Like marketing gremlins :-> ? ... but ok, there really might be hardware contraints due to the amount of preflashes, but even if not Canon almost never backports such features anyway. I still think optical fallback with a 2 or 3 group constraint would have been possible to keep on using optical flashes while rt are linked via radio at the same time.