Spray Advantage in Grand Forks, ND has a system they put together with MicroTrak that uses two electric pumps. If you need more flow than one will handle, the second pump kicks in. Controlled by a MicroTrak Electric Motor Driver. 

Here is the literature from Micro-Trak on it.

https://micro-trak.com/download/system-diagram/electric_motor_driver/system-diagrams-electric-motor-driver-pwm-two-pumps-low-load.pdf