Just an idea from someone with no claimed knowledge. If pack A has an useable volts output range of say 350 to 400v and has its own charging and BMS (protection and balancing) controlling its output contactor then through a power diode to a central junction. Pack B has the above but 330 to 380 range...

Hi, first post from a frequent lurker. I thought this looked attractive. I'm not quite at the stage of buying parts. Auction ends in 1 day so not much time to register to bid. https://auctions.hills-motors.co.uk/vehicle-list/details/2017-tesla-model-s-75d-4wd-electric-automatic-1-speed-rear-axle--no...