It depends on pump flow and pressure.
I run two 6an feed lines, one per rail, and one 6an return. This isn't an issue for me as the twin Walbro 255's don't flow a huge amount of fuel. With a larger pump pressure would rise, (at idle mainly, as the injectors aren't using much fuel.)
A way around this is to reduce the power to the pump at low demand using a pwm output controller. If you can slow the pumps down enough that the return doesn't max out, and the pressure is stable, and tune it into the map somehow you will be able to run a massive pump off a small return. I have a 1500hp Fuel lab to fit into my setup for this very reason, they have a built in PWM controller which can be speed controlled by any ecu.