From the Mega Flow web site:
So if 1 overflow can handle a 2400 gph pump with the head factored in then a dual could certainly handle a QO 9000. I have a single overflow on my tank and run a 1000 gph pump with no issues because of the head height.
I am sure your Iwakis are very quiet but Quiet Ones aren't always quiet. I run a Cal Pump 900 on my tank and you can't even hear it with the stand doors open. But again, it's not a Quiet One like Wendy is dealing with.
Aquariums up to 110 gallons are fitted with one MegaFlow Overflow and are rated for a maximum flow through of 600 G.P.H. (Using a rated 2,400 G.P.H. pump at a 5 foot head). Aquariums over 110 gallons are fitted with two MegaFlow Overflows and are rated for a maximum flow through of 1,200 G.P.H
So if 1 overflow can handle a 2400 gph pump with the head factored in then a dual could certainly handle a QO 9000. I have a single overflow on my tank and run a 1000 gph pump with no issues because of the head height.
I am sure your Iwakis are very quiet but Quiet Ones aren't always quiet. I run a Cal Pump 900 on my tank and you can't even hear it with the stand doors open. But again, it's not a Quiet One like Wendy is dealing with.