Dan, have you considered using one TW to jump start the other? Those flat 2-pole connectors referred to as "SAE" or "Battery Tender" you likely have already wired in can be linked together with 10 or 12 gauge compatible harnesses like this: Then one bike can start the other without removing seat and swapping batteries, or using conventional jumper cables:
KUNCAN 3FT Sae to Sae Extension Cable DC Power Heavy Duty 12AWG 2 Pin Quick Disconnect Wire Harness with Waterproof
Over New Years I used similar 18 guage wiring to trickle charge my van's dead battery from a fast idling Betty Boop. The technically horribly underrated wiring did the job though saving me from dismounting Van's battery and running it into town for charging at a service station. My problems always seem to occur late on a Friday in a remote place in the face of oncoming storm. I bought a jump start lithium battery pack since and plan to make up a similar cable for starting TW with remote battery without removing the seat.