If the facilities are close together geographically, you can do one evaluation for the farm as a whole and enter the information into one of the facility accounts. For the ‘percent’ dry field, enter it assuming the dry cows do not leave the facility.
FARM ES is LCA-based, so it’s trying to get the GHGs associated with the milk. Even though the dry cows leave that facility, the environmental impact associated with caring for the dry cows (feed, etc.) should be ‘associated’ with the milk from the facility that is doing the milking.