I agree with ecc83 that a call to Allen and Heath is in order. I had a quick look at the block diagram and I'm still not clear whether you can send aux feeds for monitors to the power amp and something different to the Main L&R outs.
However, on a more general issue, I'd probably prefer a mixer that just mixes and separate external power amps for monitors and mains (or powere speakers for one of the applications. You can probably put together this sort of system for similar money to the PA20 and have a lot more flexibility.
That's what I do. I drink and I know things.
-Tyrion Lannister (and Bobbsy)