alot of them dont have any springs, they are kept shut by gravity, its a light weight door that opens one way from air pressure, you can plumb both of your fans to exhaust out of one of those exhaust vents... just attach a "Y" pipe to the exhaust vent, and attach flex ventilation pipe from the fans to the "Y" pipe. you can get them at any place that sells heating and airconditioning ventilation equipment, I believe that revy and home depot carry them too.