I have an eos 400d, a canon 580 ex and a canon 550 ex. In theory I should be able to set them up so that one of the flashes (i have been using the 550) is off to the side as the main light and the other flash (i have been using the 580) on camera as the controller and the fill light. in theory i should be able to do it but my instructions are for 580-580 and 550-550 so i have only managed it once and that was more of a happy accident than anything else. any suggestions would be greatly appreciated.