You might be able to use a solid state relay and use PWM to control a secondary pump. I'd just make sure you have valves on the outlets of each pump so your secondary pump doesn't try feed back into the primary pump and cause it to cavitate. I've personally wanted to look at using one for an electric water pump by stripping the thermostat out and pulsing the pump at 20% when cold and ramp it up with temp.
A couple of tuning shops over this way use SSR's when you only have low current outputs left over and want to say run PWM for water pumps, fuel pumps or run say 8 secondary injectors etc.... Running a secondary fuel pump with PWM, as mentioned above you'd need a fuel pressure sensor and run the secondary pump with a PID, or you may be able to just switch the pump on above a certain RPM and do some funky work in your VE table above the switchpoint where the second pump kicks in?
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