I've got a center overflow with two 1" drains and two 3/4" returns. I had Durso's on both drains...and had a quiet overflow and a noisy sump. I was lengthing the Durso's but realized that I didn't have enough pipe (1 1/4"). So after reading this thread I lengthened one, stuck the open (shorter) pipe on the other and fired up the pump.
The open drain handled the entire flow from the Mag 18, so i began to close down the ball valve until I got the water level in the overflow where I wanted it and the sump was blissfully quiet. The Durso acts as the backup--and the upper drain limit--it feeds the fuge, so I needed it to carry part of the flow. I've still got some tweaking to do, but so far I'm pleased!