I think, than the best way to re-wire will done inside of the box, so by using swith one can switch to 1 battery (opposite side of motor), zero in the midle and 2 batteries in parallel. this way you keep your battery packs not changed, can use same charger.
My personal proble is not a distance, but a torgue - ability to go uphill. I do not think parallel wiring will help with this