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