Using a battery as a voltage source in a DC DC converter to supply a second battery?
Both the Li Ion and LiPo are 4S configurations with different capacities but similar voltages so either one can be higher.  You could use a bidirectional buck boost charger or consider my design, which is incomplete.  Using 50 mOhm P FETs or lower and 20mOhm Choke rated for desired 2x charge current.

The impedance of 130uH is about 4 Ohm at 5kHz.


Choice of high or low side bridging is a matter of preference