I know you can't run AC difference phases in different pipes but what about 125 volt DC.
Hot =one metal pipe Ground = another metal pipe from metal box to metal box in EMT coming off a large batterey bank.

I don't think it would get hot but I bet you could stick your Kliens on the box and the magnetism would hold them there.

If the circuit is covered by the NEC 300.3(B) applies and requires the circuit conductors to be run together unless one of the exceptions fits.

Are you sure that you are dealing with HOT and GROUND? All of our strings are plus and minus and floating with respect to ground. Some of our apps use ground detectors while we just check others during monthly battery checks.
Joe

Joe
Your right Plus and Minus But working with AC 99% of the time I used the wrong term.