yep - right i spoke to gaz about this whose a long time cubase freak, he doesn't think that there is a proper way to do it in cubase (shocking! get logic instead:) ) but you can run the C1 comp/gate in split mode and run a mono signal into one channel and the side chain single into the other channel and use that, you'd have to split the other channel into another compress to handle a stereo mix.

alternatively you could i suppose just draw in volume automation on a bus if you've got a simple enough thing going on...