From a prior page, SU reciprocating diaphram fuel pumps are not positive displacement. The spring behind the diaphragm sets a specific limit for output pressure.

This is a bit of a brain teaser. Think in terms of absolute pressure (psia) rather than gauge pressure (psig). The SU fuel pump has a spring behind the diaphragm, and the spring produces a constant output "force". There is also atmospheric pressure behind the diaphragm along with the spring. When the pump is on the delivery stroke (spring powered), the intake check valve is closed, so the pump has no reference to atmospheric pressure that is pushing fuel from the tank. Output pressure (psia) is then the sum of pressure from spring force plus atmospheric pressure behind the diaphragm. Output flow from the pump is working against atmospheric pressure in the carburetor float chamber. Absolute pressure in the float chamber will offset absolute pressure behind the diaphragm in the fuel pump. Differential pressure on the fuel pipe (pump to carb) is then equal to presure supplied by the spring in the fuel pump regardless of atmospheric pressure.

If you drive the car to the top of Pikes Peak (Colorado, USA) where there is 60% as much atmospheric pressure at 14,115 feet (compared to sea level), the SU fuel pump will still deliver fuel with the same efficiency. It will also not create excess pressure that might cause fuel overflow. If you put a pressure gauge at the delivery end of the pipe just before the float valve, the gauge will give a read out of pressure created by the spring only, because the gauge also has a reference to atmospheric pressure. The pressure gauge in free air still reads zero with same pressure on both sides. The engine and carburetor with half as much air at high altitude can only consume half as much fuel, but the "constant depression" SU carburetor does an admirable job of mixing the correct amount of fuel for the air that is available.

On a slightly different tac, most of the electronic aftermarket fuel pumps use a moving piston with two check valves, fuel on both sides of the piston, and no atmospheric air inside the pump. This type of fuel pump will produce a fixed pressure differential beween inlet and outlet ports. If you plumb two of these in series to run at the same time it would indeed double the output pressure, so only one can be allowed to run, while the other can be used as a backup with an electrical toggle switch.

For the moving piston fuel pump at sea level, if it generates 3 psi pressure the inlet port will be at 14.7 psia while the outlet port will be at 17.7 psia. This output pressure works against 14.7 psia pressure in the carburetor float chamber, so the result is net 3 psi pressure differential pushing fuel through the float valve. Take this one to the top of Pikes Peak, with 60% as much atmospheric pressure, and you have 8.8 psia inlet pressure and 11.8 psia outlet pressure. This is working against 8.8 psia pressure in the float chamber, so it still generates net 3 psi pressure differntial for pushing fuel through the float valve. This also works just as well at high altitude.