In my Flash I have 2 A123's going into an orbit power jack. I am using 2 Smart Fly Heavy duty switches. In order for me to charge both batteries at the same time I need to disconnect the batteries from the receiver. Or my Cell Pro 10 tells me "Reverse polarity".
But then again they charge so fast so sometimes I don't bother.

In my Boomer I have 2 A123's going into a smart fly competition 12 with the flag and pin switch.
I can charge both at the same time with out unplugging anything