again: RTFM!
Check page 13:
http://www.allen-heath.com/media/Qu-Mixer-User-Guide-AP9372_1.pdf
Main L&R are on the Channel 11 and 12 on a AR2412 or on 3+4 on a AR84!
So you don’t need to USe Mix1 and Mix2 for Main L&R Just Plug the PA in to the correct Channels of the AR Rack!
