Hi David
I know my way about auto elictrcal systems but I am in no way an electrical theory whizz kid.
I would imagine that you could use two strands of wire.
Personally though I would prefer to go through the hassle of feeding a new, heavy gauge wire from the bulkhead & make use of a decent relay.
Most external electric pumps I have seen have been laying flat, that said all of the in-tank pumps I have seen are upright
Probably both work in different ways.
Hopefully someone will come up with decent answer to your questions mate.
Probably, I been as much use as an ashtray on a motorbike on this one
