I like DvS's thinking there, its a bit cleaner and only one pump which is nice. The ball valve should let you adjust how much of the over-flow you want to run through the fuge and how much to the sump.
Also, Birdy, on your drawing, I am not sure you need the ball valves on the over-flow pumps. I would only put the valve on the pump return to make sure your not over-pumping the tank, and then just let the return rate be dictated by the pump rate.
_Scott