I changed for testing  one of my Z9 mini to dual fan.
https://image.ibb.co/gQhXXp/mini_edit.jpg

Maybe later I will cut a big whole in the backplate.
https://preview.ibb.co/my7hXp/IMG_20180828_190527_HHT.jpg

Stats of the dualfan Z9 mini
https://image.ibb.co/cF1ZCp/mini1.jpg

Stats of the normal Z9 mini
https://image.ibb.co/jEPXyU/mini_2.jpg

EDIT
Some thoughts, the big advantage is that the miner have now enough power to push the Air out. My problem is that there is no good way to install the hose. Does anybody know where to get a backplate with screws for the other side.

The hose (I'm not sure about the right word in english)
https://preview.ibb.co/kNWFk9/IMG_20180828_200519_HHT.jpg