Strobist: Cross-lit with two double (4 strobes in total) 580EX. By accident one of the double didn’t fire on some of the images. Flashes set on ¼ power. Thus doubling that would be the same as one strobe on ½ power. With ¼ power the strobes can fire as fast as my camera. (if the battery pack don’t get too cold or exhausted).