I sometimes use 2 Metz hammer head flashes with umbrellas I have one on a stand with a coiled 15ft flash extension lead attached to the camera and the other on another stand with an optical slave trigger attached, I plug the extension lead into my flash meter to take an exposure reading, and then back into the camera. The multiple flash adaptors to fire several flashes to one camera sync socket we used to call Sputniks in the old days can damage the cameras flash sync ability.