Drizzt321 said:
I'm not sure you can stack ETTL cables, or chain them like you would need for this. It'd be an interesting experiment though, although it'd cost you a bit of money for the ETTL cables you'd cut apart and solder. Anyone want to sponsor me? I'd need a couple of Canon speedlites, as I only have 1 right now.
Classic shot by the OP ;D. You wouldn't be able to stack ETTL cables, they are an SPI like interface where you have a a master / slave type arrangement. When the master (the camera) sends the commands to the slaves (flashes) they would all try to respond at the same time. When you want to stack that kind of interface you normally need a seperate chip select type line to force the slaves into a high impedance which Canon flashes don't allow for from my reading.
Only way I think would be a microcontroller that sat between all the flashes and the camera and actually interpreted and tried to respond intelligently to the requests, like maybe firing a pre-flash from all flashes for metering but then only returned metering information back from the central one so the camera thought it was dealing with a single unit.