With your Mallory 2-port regulator, you should have 1 inlet port, 2 outlet ports, and a 1/8" npt port for a pressure gauge. None of those are return ports. If you unplug the other outlet and return it to the tank, you'll be dumping most of your fuel pressure and will definately lose performance.
I always run one outlet to each float bowl on a Holley.
Johnny