When i ran an electric pump. i left the mechanical one in place. The fuel passed through both of them. I added the electric one when i had a small vapour lock problem one hot summers day in my 1000 sedan [with 1200GX] There was 4 of us in the sedan, towing a boat, which was loaded with all our camping gear & the outboard motor, as i was about to pull out to pass this slacker in a Commodore while coming up a steepish pinch on the Clyde mountain. The engine died for a second or so, then got with the programme. Those 5 speed option 1 boxes are magic.
The electric pump helped out with this problem & it never happened again.
Chris

interesting. what pressure is the pump running at? does the mechanical one act as a kind of regulator?

The mechanical pump is a true GX one & these run at a slightly higher pressure than a stock 1200 one. The carbs would see the pressure of whichever pump put out the higher pressure as the mechanical one has two simple one way valves & if the electric one was higher [which i seriously doubt] the fuel would simply flow straight in & out of these two valves.

The electric pump was fitted so as to push some liquid fuel into the mechanical one in the event that it got hot enough to form a vapour pocket inside the pump, & this is what stops the pump from pumping. When this happens, the carbs run out of fuel, but the electric pump just pushes the fuel straight through.
I was in luck that day as just enough liquid fuel got in & it pumped again, & with the high fuel usage at full throttle, the pump became "fuel cooled"

Simply put, i raised enough energy to fit the electric one, but was too damn lazy to remove the mechanical one. I did not have a blanking off plate & could not be bothered to make one. It worked fine.
Chris