Hi Ian,
As discussed in a separate thread, Dual Surface operation on a single MixRack is not yet implemented nor announced as a feature. All I can say at the moment is that we will be looking into it. The preamp would be shared. Digital Trim on the Input channels would be independent for FoH and monitors, same as with two complete dLive / iLive / GLD systems.
For the time being I would recommend planning for two separate systems (MixRack + Surface) for FoH / Mon applications.