I should be able to use a Cactus unit on my 500D hotshoe (w/o a flash on it), set it as my transmitter, and then have two receivers, right? So I'm triggering two 580's with one transmitter. I can only get one flash to fire. When I press the little "test" button on the transmitter, both of the receivers light up green, so they are all talking. All of them are on channel 1. I can press the "pilot" button and get the non-working one to fire, so it's not a battery issue or anything. Any help?