Here's my understanding of the original 2-battery circuit. You may have to pick a different MOSFET. I simulated the functional circuit with a 10 ohm load, and it seemed to work well. If the voltage on V4 drops below about 5v, the circuit will not work.
When the input is 0v, M1 is off and M2 is on, and all load current is provided by V3. When the input is 9v, M1 is on and M2 is off, and the load current (and voltage) is provided by V3 and V4 in series.
If this isn't what you want, let us know.