This is version 2 of my 2 flash macro setup. This version uses a Bogen/manfrotto macro flash bracket. I connect one flash to the camera with a Nikon SC-17 off-camera flash cable (which supports all flash functions) and then connect the second flash to the first with an ordinary PC sync cable.
I frequently use the first flash on TTL with about -1EV flash compensation dialed in, and set the power of the second flash manually. The TTL on the first flash compensates for small changes in flash-to-subject distance, which can still have a dramatic effect on exposure at such short distances (The difference between flash-to-subject distances of 1 and 1.4 inches is a full stop of exposure!)