Charge and Balance x2 batteries on a single Charging port with Speedy RC's Dual 2S 4/5mm Charge Lead. This lead simulates a 4S 14.8V (or HV) LiPo battery setup. Set your Charger accordingly and you're good to go!
Charger:
Set your charger to 4S LiPo. Connect the 4mm banana plugs and 4S balance connectors to the charger.
Battery:
Ensure the battery's are the same capacity (mAh).
Plug the 4/5mm bullet and balance pin connectors into your batteries noting white collars.
White collars in first battery, no collars in the second battery.