I do most of my lighting using ETTL. I'd like to add another flash just for background gels. I'd prefer not to spend the money on a Canon flash if I'm just using it for BG lighting. So can I use the ST-E2 (mounted on the hotshoe) and use the PC terminal to trigger the BG flash at the same time (maybe using the cheapo Cactus triggers)?
My other option would be to use an optical trigger, but I'm not sure if there's one that works well with ETTL's test flash.