Now this is embarrassing! I don’t have any circuit information on the VC1 version with the stereo link! But I think you will find that the co-ax socket is just an extension of the rectified sidechain, so connecting them together will allow both units to control the compression. I can’t be absolutely sure about that (without having one of the units here) but I do remember that it was not a successful facility owing to the fact that the opto cells vary so much and the compression was never balanced. As you noticed, we very quickly dropped the facility!