Re: common GES
I don't believe the two referenced sections above apply. You appear to have two separate services serving two separate "structures" that happen to be right next to each other.
Each service is required to have a grounding electrode system. I don't believe there is anything in the code that prevents you from using the same grounding electrodes, however, I personally don't think its a good design.
If you want to get real technical, I feel what you describe is a viloation of Section 250.6 The GES as you describe bonds the 2 separate service grounded conductor together and provides a path for return current to flow on non-current carrying metal parts.
I would design two separate systems, meaning 4 ground rods and isolation between the two by distance. Alow the Earth to become a "resistor", limiting the amont of current exchange between the two serivces.
