If you insist on using the batteries separately I would recommend something similar to this, although it can also be done with N-FETs on the low side

^{simulate this circuit – Schematic created using CircuitLab}

If you insist on using the batteries separately I would recommend something similar to this, although it can also be done with N-FETs on the low side

^{simulate this circuit – Schematic created using CircuitLab}

The schematic below should work for inverting the output signal for control of the primary pack. The actual values for components should be evaluated based on actual circuit needs(Gate voltages on specific MOSFETs, filtering needs, etc.)

^{simulate this circuit}