Lloyd,
I can't visualize what you're contemplating, but if each pump has a mechanical pressure regulator and they both power the same rail I'm not sure how you balance the regulators so that one doesn't open earlier than the other and feed all the fuel to the tank controlled by the weakest regulator.
I also have the Inglese/FAST system and you may already know, but you CANNOT run lines from both sides of the rail as the distributor is in the way on the firewall side. As such, if either pump grabs air that air can ONLY escape though the injectors (a design flaw versus a TWM system IMHO that could be configured to allow the air to pass through the regulator.
Also, most high pressure pumps must pass fuel all the time or they'll overheat. I'm happy to discuss if you'd like as I probably spent 40 hours contemplating the fuel delivery system on my build. Admittedly I over-engineered by system but included a PDF configuration of a pretty simple surge tank system for EFI that I clipped from another thread.
Finally, if you need a quality return valve, I have another marine valve (see picture 2) that I'd be willing to sell you if you'd like. It was made by Parker Hannifin (
Parker Marine Selector Valve Specialized), has one in and two out powered by solenoids, with a manual override (red handle). It is a lot more expensive than the Pollack valves, but are 1/2 NPT so they flow a lot more.
I basically wired it from the respective fuel pump so that fuel is directed back to the pump that is active.