HELP!! I need to be able to fire my SB600 simultaneously with my SB900 OFF camera. I am using a sound trigger to fire the SB900 flash in commander mode but the SB600 does not also fire. Can anyone please tell me how two set the two flashes up for this function. If I place the SB900 ON the camera everything works as it should. I have tried reading the manuals but they only seem to give instructions for ON camera use, nothing specific for OFF camera. Any help would be greatly appreciated.