Bobbo wrote:
Be sure each wire, alone, is sized to carry the entire load. That way, if one of them loses connection, you won't have a fire hazard. The second wire in that scenario is merely to reduce voltage drop.
You can also fuse each wire so if it gets all the amps, its fuse will blow.
If a fatter single wire would need a 300a fuse, you can put a 300a fuse on the positive battery post and run two thinner wires to that fuse, or you can put a 150a fuse on each wire and put the two fuses on the battery post.