Using 2 580EXII's with ETTL II for more power/quicker recycling

[gladivs]

I'm going to be at a fashion show where I'll be photographing models walking down the runway.

I don't know the specifics of the lighting but want to be prepared. 

I have previously had success with using a single 580EX on my 5DMk3.  However, I had to go a little higher with my ISO that brought in more ambient than I'd like. 

I have Yongnuo 522C transceivers and was wondering about using a 580 on-camera as well as one off camera, directly behind me, to give a little extra power or to speed recycling.  I would likely put both on the same A group.

Any idea if this will screw up the flash algorithms?  I normally like to test this first but not sure I'll get the opportunity.

Thanks!

[kvbarkley]

That is the whole point of wireless flash, that the camera fires a pre-flash from all the flashes to be used and determines the proper exposure.

I know it is more expensive, but I suggest an all-canon setup with a 600RT as the master (in optical flahs mode) on the camrea and the 580EX as a slave.

But having the light come right from the camera is not a good plan, usually.

[diverhank]

That should work very well using both in ETTL.  Regarding your statement of not letting in too much ambient light.  I'm the opposite...I will use ambient light as much as possible and use flash only as fill and not as the main light source - I just don't think the pictures look very natural when flash is the primary source of light.