I can tell you that we wanted to do this exact thing on a remodel job so we asked the inspector. Turns out you ARE allowed to do it but there is a formula you use to determine the size needed for the final duct, and it IS bigger than the two from the fans. The dampers should prevent any noticeable backdraft into the other fan.

Anyway since we are not HVAC technicians we ran separate ducts out the roof.

Which brings up a question, how many of you will provide the exhaust for the fan when there is no HVAC on the job?
Or do you call in an HVAC tech and a roofer for a fart fan?

Last edited by BigB; 08/14/13 11:57 PM.