450D & 580EX II off 12V battery
What adaptors and cables would I need to connect two Canon 580EX II
flashes to a 450D. The two flashes must both be controlled by the
You will need nothing more. You can put one flash on the camera and set it in Master mode and the other on a tripod in Slave Mode. From your camera you can control the A/B ratios of the two flashes. Start reading here: http://photonotes.org/articles/eos-flash/
But, if you want both flashes off camera and don't care about eTTL then you'll need a hotshoe to PC sync adapter like this one:
and use cables like these:
to connect from camera to flash.
In that case start reading http://www.strobist.com
Alternatively to use both flashes off camera in eTTL mode you'll need the Canon Speedlite Transmitter ST-E2 (pricey). Check photonotes.org above for details.