Two in the same overflow will fight each other, but it should be doable. 

 Alternatively, you could remove the pipe from the drain going to the refugia and use the ball valve to adjust this flow.  The remaining flow would be directed through a single duro to the sump.  Or vice versa.

If you can fit an airline in the duro hole, it's too big.  I have dual overflows with a durso in each one.  My return pump is a Velocity T4 (1400 gph) and I have no valve on the outlet.  I can barely fit a toothpick in the hole for the durso.

The tee with cap is a very good set-up.  But don't drill a hole in the top of the cap, drill a 3/16" hole in the side through the cap and pipe.  Then you can just turn the cap to reduce the hole size.  This makes it very easy to dial in the right about of air.

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.

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

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.

Put the durso on the drain going to the sump.

http://www.thinkreef.com/reef-aquarium-sump-plumbing/ - 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.htm - http://www.rl180reef.com/180/pages/standpipe/mods/external.h tm

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.

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