Steve,
I used a stock Mazda 1986 fuel pressure regulator, two NAPA 2P4028 fuel pumps in parallel, and an Andair duplex fuel selector valve. The fuel lines are -6 (3/8") except that the fuel return to one of the tanks has an additional 3 ft of -4 (1/4") aluminum tubing. The fuel pumps each move 47 gal/hr against a 7 psi head.
The fuel pressure in the rail is 38 psi when either pump is running. The current draw from either pump is 6 amps.
When both pumps are running, the fuel pressure in the rail is 39.1 psi when the tank without the additional 1/4" return tubing is selected and 39.6 psi when the tank with the additional 1/4" return tubing is selected. The current draw from both pumps is 12 amps.
Since with my setup running both pumps only increases the pressure by about 1 psi, and the 3 ft of 1/4" tubing only adds about 0.5 psi of back pressure with both pumps running, it might be worth checking for a restriction somewhere in your system.
Steve Boese