A shorting A/B switch, meaning B is connected before A is released, would allow trickle charging both batteries to the 80% you suggest.

Leno's video brought up the battery life question. They are all doing well as long as the charge is held between 30% and 70% but no one knows how they will hold up if, as Ford's head guy suggested, the PHEV Escape is charged at night to 100%.

I believe Li batteries can be gauged from a precise voltage measurement. If so, the two batteries in parallel would give a greater depletion mode run.

BTW. I believe you are in AL, so you probably know where we retireed--on top of Lookout Mountain, near Mentone. One reason, I would like a larger battery is the 6 to 8 mile ride down the mountain. I don't think the stock battery can store all of that energy.