If you run a return line, how do plan on returning equal parts to both tanks? You could to do as the OEMs did, use a fuel diverter valve and run off one tank then switch to the other.
Diesels trucks have both tanks plumbed together, so the level in each is always equal. This would let you simplify the return plumbing and supply hoses. If the connecting line was big enough, you could fill both tanks from stock location, assuming they are mounted at equal hight. This would also let you use only one sending unit for the fuel gauge.
