I did 2 dursos in the the same overflow.  I'm not sure if they each get exactly 50% of the water, but I think how I designed them works well.  Where the "T" is at on each durso, replace it with a cross "+".  Then connect one side of each cross with a short piece of PVC (the length of it will depend on the spacing between the standpipes).  By doing it this way, the partial siphon pressure inside each side of the dual durso is equal.  At least that's the way I think it works... all I really know is it works.

Steve

If anyone is using a durso on a hang-on-back overflow I would love to see some pics of how you did it.  I would like to quiet mine down a bit this way.

I just built a stockman standpipe for my overflow.  Will work on some pics for you.

Put the durso on the drain going to the sump.

http://www.thinkreef.com/reef-aquarium-sump-plumbing/

It just need to be on the drain side of the outside box's weir.

http://www.rl180reef.com/180/pages/standpipe/mods/external.h tm

Will my single 1" durso be big enough to handle 900 - 1000 gph?

Yes.  My father-in-law used a 1" durso with a Mag18.

Jon, I have the same situation for the new 90g.  Will my single 1" durso be big enough to handle 900 - 1000 gph?  Then I could just put the gate valve on the drain and head it off to the fuge at whatever flow rate I wanted.

Thanks,

Fax

Well, if I take the durso off the refugium pipe, wouldn't it be just as loud?

No, you'll pinch in enough on the valve that the water level in the overflow box will be up at the level of the othere (durso) drain.  The valved drain line will be liquid full and should not be noisy at all.

I initially had a much smaller hole, but it didn't get enough air so it would siphon really quickly until the water level got below the durso, then flush, and repeat. 

Yup, that's a sign that the hole was too small.

Good luck.