Many years ago I started to put together a fuel system similar to this. My plan was to run a second 3/8” fuel line that started inside the tank. My plan was to run a fuel pressure sensor in the original fuel line. Once the pressure dropped to a certain point the electric fuel pump would energize. The fuel line with the electric fuel pump also had a check valve in it. My reasoning for dual fuel lines was that if the pumps were in series and the vanes in the electric pump could block or part block off the flow of fuel. The other thing is that the check valve would stop any fuel from the mechanical fuel pump to back feed through the electric fuel pump while the electric was not running. I never followed through with it though and I may have over complicated it.