Seems straight forward to me.
Guitar to the GCX switcher (with pedals in loops)
GCX to the Mesa Amp Switcher (with 50 cal + and Roland in the loops)
Mesa Amp Switcher (MAS) to Cab
With that kind of set up, you really can't run the effects after your preamps since you can't use the amp effects loops if you want all the pedals to be access for both amps.
If you are only using certain pedals for each amp I would consider two GCX units and running each in the loops of the Mesa Amp Switcher in front of the amps (or through the amps effects loops if that is how you get the sound you want). If you tried to do this with one GCX I would bet it would result in grounding issues and plenty of unwanted noise. If you would only use the pedals for one amp, say the Roland, I would run the GCX in the MAS loop with the Roland. This way when you use the 50 cal the guitar signal will not go through the GCX as this could impact the feel/tone of the amp. Pickups react differently when going directly into a preamp vs. through a pedal/pedal board and then to the amp. Going through the MAS may have the same effect though. The MAS is one Mesa goodie I have yet to try myself.
Hope I helped. If you have any other questions, let me know. I am not a rack expert but I do use rack gear on a regular basis.
