Hi,
Looks like you do need two separate outputs to independently drive the two pumps at varying power levels.
Since I am controlling 220V, I want to minimize the possibility of outputting anything but 50hz or off.
I would suggest you AC couple the drive to the mosfets to give you some hardware 'insurance' against leaving one or both on all the time.
Driving at 50Hz gives you quite a bit of time ( 10mS on, 10mS off), you could do that by timing the outputs using millis()... have a look at this very simple Blink Without Delay example
Yours,
TonyWilk