If you have the space on the surface, you just have to address the 2 separate strips to the same mix rack input.
The split occurs after the pre-amp module so any pre-amp adjustment affects both channels.
Due to that, I would advise that you set only 1 channel with actual pre-amp control, or leave the pre-amp set and set both channels with digital trim only.(this is a setting on the pre-amp parameter screen per channel or system wide on all channels)
I suppose I should have asked what are you working with, surface, rack, firmware version, single/dual rack setup, etc,,,,,