Did you find a solution for the Ultra2 to be able to hotswap? I'm right now looking around for a suitable solution.
Except by doing it that way you will also try to charge the battery that has a lower voltage.
You should be fine as long as you connect Batt A's +12V to Batt B's +12V and Batt A's -12V to -12V of Batt B. This puts them in parallel which will double your Amps.
I think Eric is referring to the magic diode to prevent reverse voltage.
Just got the good/official news from Robert Orf over at Tiffen. After extensive testing, inserting a 3rd battery in 24V Parallel mode via the Ultra2's base 3pin accessory power port (Pin 2), by placing a diode in series on the plus side (+12v) will protect the battery from receiving a charge and will not affect the DC-DC converter.
The diode tested by Tiffen is DIGI-KEY part #90SQ030-ND
Thank you to Robert Orf and his team at Tiffen/Steadicam