Working with a friend on a stock 71/72 240Z. It currently has both an aftermarket electric fuel pump and a mechanical pump. Both appear to work. Can the manual pump work if the electric is turned off and or the other way around.

In theory... Yes.

Both fuel pumps are positive displacement types and have check valves on both the input and output sides. Because of those check valves in theory, either pump will allow draw through with the pump not on operation. So, you could pull fuel through the electric with the manual, or push fuel through the manual with the electric. In theory.

Problem though is the amount of pressure drop you would get across those check valves, and would the system still be able to build the required pressure in the rail.

In theory.      Haha!