Fuel line size (help Jody)

[69x22]

I am running a F1 Pro Charged 417 stroker. The fuel pumps are 2 Walboro 225s, mounted separately in a SS tank. Plans are to run the second one on a micro switch off of the accelerator. Questions are what size supply line and what size return. I will be running a Aeromotive bypass valve up by the engine.
Also when 1 pump is running what will keep it from back flowing into the second one? Do I need to install some sort of check valve?
Thanks