For microphotography, I use two 600EX-RT flashes. I don't really care how, but I want to fire them at the same time using second curtain sync. Second curtain sync is crucial for me. Canon does not support this sync type when there is radio or optical communication either between two flashes or between the radio transmitter and one or mor flashes.
Therefore, I have connected one flash using a hotshoe extension cable and the other is connected either via the hotshoe or via a PC sync cable to a Hama 6977 slave unit. Immediately after the first flash is fired, the flash connected to the Hama slave unit completely freezes and it does not respond to any buttons whatsoever. The only way to "fix" it is by either detaching and reattaching the Hama slave unit or by switching the flash off and on again. I experience this with the second flash in "normal", slave or master mode, but I need some mode where I can manually adjust the flash output (which is not possible in slave mode).
What am I doing wrong here and how can I solve this issue? Any suggestions are welcome. Thank you!