That's a good issue. It would be good to get some plumbers' perspective on this..tho sometimes its hard to separate folklore from fact when talking electrolysis.
I wonder if the diciding factor should be whether or not the separate buildings are all fed from the same utility source.
More to the point, two buildings with power coming from the same transformer would effectively be paralleling their grounded conductors when the services are linked through the plumbing. Conceivably, one neutral could open and be fed entirely through the other neutral! This would be a smoker, if not a voltage drop hazard to personnel.
