If you just want to side chain a stereo signal it's not too difficult, however you need a plugin that can be sidechained like the Voxengo Crunchessor. What you do is put the plugin in a Quadro group channel, route the signal you want to compress to the 2 front channels of the group and the route the keying input to the 2 rear channels by way of the key channel's surround panner. Press the sidechain button on the crunchessor and you're on your way.