Stereo out AND a mono monitor output from MainStage

Hi,

I have an audio interface with 2 main stereo outs and 8 additional outputs. Is there a way that I can duplicate outputs 1&2 into a mono output via MainStage or audio midi setup to go through the audio interface on output 3?

I know I can set up a bus to send the audio out to an aux channel routed to output 3 but that needs to be done for every instrument in every patch but I have over 40 patches and some have 20 layers and splits in the them, so doing it on every channel would take ages. I would also need to set these up as post fade, not pre fade in case we alter any volume/balance of patches.

All I want is some way to send a mono mix so it can go to an onstage amp (no audio out on it sadly) whilst the main 1&2 go to the main output a the sound engineer.

Any help would be gratefully received.

Neil