As I see it, you have several issues to deal with.
1. 400 ft between buildings means only one thing, different power sources and grounding, so you must use the same power source from the interface to the other building and back. You CANNOT use power from both buildings !
NEC 70 - cannot quote book, chapter, and verse.
2. a distance of 400' between buildings means 400 ft from building A to building B. You then go thru the isolated output, then 400 ft from building B back to building A. A total of 800 ft. You must size the wire to account for voltage loss, I would say 3 or 4 wire sizes, my rule of thumb, 1 size for every 200 ft of wire.
3. wiring must be run in its own conduit.
4. wire color must me orange to signify interface wiring and power from an alternate source. See NFPA79.
5. alternate power source placards must be on the plc door.
some rules may have changed since I did design on that scale.
if I am wrong, someone please correct me
regards,
james