On the 14, one line is suction (to the pump) and the other is return from the pump/fuel rail (excess pressure) to the tank. It was designed to keep fresh fuel in the fuel rail system on injected cars and prevent vapor lock. Of course Miles doesn't believe in fresh fuel
If your gonna run carbs - you only need one line. You can purchase a holley fuel pressure regulator and run a carbed engine like an injected engine (with a fuel loop). Carbs need only 3-4 psig, which is about what the FACET pump puts out. Anything over 5 will collapse the floats and cause flooding (can you say FIRE). If you run a high pressure pump, the excess pressure is bled back to the tank thru the return line.
There are diagrams somewhere on this site and in a Haynes Manual or ?
Race cars (which Miles doesn't have) will run multiple pumps because if one fails ya got another - this would be the only logical reason. That said, some cars will run multiple pumps - an "in tank pump" to get the fuel over the hump to feed another electric high pressure pump that feeds the engine - Can you say Volvo.
Damn, this is really to serious! I gotta go get a Billy Beer.