The two mono groups option could kind of work, but it could be awkward to have to use the faders to pan a mono channel between the two groups to get your stereo image. What you could do is use a stereo subgroup (not routed to LR,) then have a couple of spare input channels set their source to either side of that subgroup. You could then compress each of these channels separately, and route them to LR, panned hard left and hard right. IIRC as of 1.3 you can do this without needing to physically go out of the AR and come back in, so everything stays in the digital domain.
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