Well if you're really concerned with amperage just use the reds together and the blacks together, using them in parallel doubles the amperage capacity. Mechanically join them at the source and destination, use a short jumper to your plug. After all, once the potential gets to the plug and goes to the T/M, the maximum wire size inside is #8 if that.
However, I like simplicity, and some might find rewiring a boat an appealing pursuit.

The wiring you are considering is almost a half inch in diameter which could give you fitment and radius issues.