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Gregg Heckler wrote:
...A lot of your success will depend on how much ambient light there is using only two small flashes.
I wouldn't call two Alien Bees B800 strobes "small flashes." They're not Dynalite 1600 w/s packs or the like, but they're a definite step up from Speedlites.
...You need to consider the inverse-square fall-off from any light placed near the camera; it will light the front row brighter than the back row.
With 100+ people in 4 or 5 rows, the camera and lights will likely be far enough back that the difference in intensity between the front row and back row will be negligible. Bouncing off the ceiling, on the other hand, can be problematic if the ceiling height isn't right, if it's not white, if there are beams, etc.
Unlike shooting -- for example -- a bride getting dressed, where the room is usually small and bouncing works well, I've found that for large groups in convention halls, arenas, etc. direct flash usually works better.