To use two stock hybrid batteries you would need an A/B switch... basically use one at a time. I don't think it has been tried in a Ford. There are so many communication wires/monitors involved, that I don't think it is feasible to just connect the high power lines in parallel. Plus you will have all that extra weight, and not much more EV time.

Did anyone else catch on from Jay Leno's Test Drive that Ford's PHEV has THREE separate liquid cooling systems? Catch the thrid green fluid container on the firewall that sounds like it is dedicated to the transmission. So Ford's PHEV has:

1) Liquid cooled ( via radiator ) engine loop ( yellow )
2) Liquid cooled ( via radiator ) DC / DC converter loop ( yellow )
3) Liquid cooled ( radiator ? ) Transmission loop ( green )
4) Don't know if there is still an A/C freon loop to the HV battery ( likely )

They say the LiIon battery is "280 volts".
They say the 10kW battery recharges in "6 hours" with a 1500w plug-in.
That is only possible if they use less than 80% or less than 8kW of the battery.
They say they take the battery to "the top" or 100%. So perhaps they never take it below 20% SOC.

Also... the hybrid display was not working correctly on the test drive!