You should also be able to do this within your DAW without Focusrite control. I only know how it works in Ableton, but there you can just connect one or two mono cables to outputs on the back and tell the audio track with the mic on input to be sent to this output rather than to the master. All you need to do is to activate all of your 18i8 outputs in Ableton audio settings. I guess this will work with every DAW. You can then plug these cables into DT and turn up external in volume for a pattern under master control page.