Well, I am currently running (1) Milwaukee pH controller. It is connected to my 75g tank. Sitting next to the 75g is the wife's 40g tank. There is no pH controller on her tank.
I have one cO2 tank with a very nice regulator and dual solenoids/metering valves. It is nice having the pH controller to see how the pH level is doing, but, now that I have gotten comfortable with what each tanks bubble rate looks like I find myself checking her tanks pH level less and less. Having the safety factor of the pH controller is nice, but I am now leaning towards setting the controller .2 or .3 lower than I actually need.