Digital trim is a network function, which is not available on a single mixer, because inputs are analog. USB sources are computer sources, mostly tracks or playbacks/streans, so gain settings have to be done on the computer (track/output levels).
When two channels share the same analog input source, there is only "Make Up Gain" available in the compressor, which allows increasing the level of the signal. There is no feature allowing to REDUCE levels. Is there really a need to use two different gains? Wouldn't it be better to add a second rack mixer as dedicated monitor mixer?