I have separated them and what i had found is as follows: it seems to work with 2 S9's exhausting into a 6 inch round duct or 1 S9 exhausting into a 4 inch round duct. That seems about the max that you can exhaust into each duct.
Thanks for all of your help and pointing out the obvious to me.
I have 5 miners that I use 6" duct on. I found my S9s are basically unaffected by a 6" duct. Anything below that and they start to rise in temp (not overheat, but just a higher running temp). My S7 can get by with a 5" duct no problem. I wouldn't even think of anything lower than a 6" duct for my firebreathing M3s
I post this link a lot, I've been in your shoes. https://bitcointalksearch.org/topic/ducting-2796117