From your AB168 rather than to it.
In the i/o patch, you can have the 16 inputs of you AB box go to (for example) 1>16, then duplicate this on 17>32. If you also have things plugged into the desk directly, you’ll have to use a physical splitter lead for those, and probably cut down the number of monitor input channels you make.
You can then use the first 16 for FOH, with gates, PEG, comps as required, and the second 16 (apart from gains & phantom) can have a different set of settings and be sent to monitors.
If I do this I turn those monitor channels off (“Assign” & “Select” buttons) to the main L&R, and then run monitor sends post fade (“Pre Fade” and “Select” to make them post) from those channels.
I often then, space permitting, put those monitor input channels into the custom layer on the same faders as their FOH ones, along with all the Monitor Masters, and also any effects I’m dedicating to monitors. So the custom layer will pretty well act as a separate monitor desk.