If you used a dual pickup system 1 plumbed to the electric pump 2 and one with a one way valve on the pick up that goes to a Y then forward to the mechanical pump. That way the mechanical pump can suck through the one way valve line around the electric pump and when the electric pump is turned on it will pressurize the system forward to the mechanical pump but wont push fuel back to the fuel tank because of the one-way valve. But thats a lot of extra plumbing to do the same thing an electric pump will do by itself.

Plus if the mechanical pumps diaphragm ever ruptures while the electric pump is on it will start to dump raw fuel directly into the crank case!!!