I have the Canon 580EXII and a 420EX. I have a low light indoor wedding ceremony coming up and what I'd like to do is shoot with my 580EXII on camera with a lightsphere and set up my 420EX to fire in the background wirelessly. Is this set up possible with a wireless transmitter? Can I have the 580EXII on camera?? Or do they both have to be off camera? Do I even need the wireless transmitter?? Can I use the 420EX with my 580EXII or does it have to be a 430EX?? Help!