[reheat module]

Picture the scene:
A small dining room studio set up, with 2 Elinchrom D-LITE 4 softboxes, working off a radio trigger (top mounted on a 5Dii), with a single sensor at the rear of one of the blocks on the power input picking up the signal, and the second elinchrom block selected to 'slave' on the rear panel. This illuminates the subjects fine - no issues.
I have a white background roll situated some 2 - 3ft behind the subjects, and understand the fall-off will make it grey (which it does rather well).
I also have a 580EX and a 580EXII - what do I need to simply have these 2 flashguns light the background only?
Would a simple 'sensor cube', individually mounted between the flashgun body and the flashgun foot on each gun pick up the light and blip the background?
Do the guns need to be set manually or ETTL?
Open to advice, guidance and experience.