I do not like dual battery systems. There is more to go wrong. Simplicate and add more lightness! Just pick the correct battery size to begin with and install a battery monitor if you are marginal or have doubts. I do this on my helis with digital servos because the current consumption is huge and variable depending on what kind of flying you are doing.

I would rather fast charge between flights than rely on seperate batteries. The model shops will advise 2 batt setups for obvious reasons.

TWO EXCEPTIONS

1 If you are using mechanical retracts then have a seperate battery that feeds the retract servo only. Eventually most people get a retract jam - stalled retract servo - very high drain during the flight.

2 If you have a split servo system with TWO rxs and the ability to fly the model on either RX then obviously having TWO batteries one per RX is a safety feature.

48 years of radio flying.