The last grinder I installed was a duplex system with a 48"x60" basin. The pumps would alternate in operation as to keep both pumps exercised. This system was a 4 float system,consisting of on,off,reset and high water.Each pump has its own stop valve and check valve and connect to a common 1.25" tee and then discharges out of the tank 1.25".
I was specifically instructed by the design engineer not to exceed a 1.5" pipe diameter and doing so would increase the head pressure on the pump.
Basically on a high pressure forced sewer your pump has to be strong enough not to just have enough head pressure to pump it to the city......your pump has to have enough power to exceed the head pressure of the citys main also and that can have 75# of head pressure. I had one that exceeded 125 pounds and the city had to add another lift station to relieve the pressure so we could get a pump that would be powerful enough without spending 3 times as much.