I tried it once but it made more noise because although the same size 80mm fans, both pushing air in the same direction, because the blades were not teamed ( physically joined ) wind sheer happened at the close air gap between both fan blades, as well as the plastic frame of the sandwiched muffin fan and it made a humming noise as one blade was sheering off past the other fans blade as well as the front fan blade was very near the backside plastic frame that holds the guts of the other fan center to its frame.
I suppose if you could find 2 fans that spun reverse direction to each other you could team them by a perfectly centered epoxy to join both exposed fan faces and this way they would be physically teamed and would be quieter than that of 2 fans that with an RPM difference you get the noise of the opposing blades sheering the air between their air gaps.
Most computer fans I have come across spin in the same direction, and its not as easy as reversing the polarity to the fan because 99.9% of the fans out there are brushless and PWM is used with a circuit board that requires power only in its correct polarity and this board pulses fixed magnets in the inside of the fan blade from a set of coils that are in close proximity with the fixed magnet to get the fan to spin. Reversing polarity will cause a smoke show or will kill the fan.
If your looking for good airflow and quiet your options are large low rpm fans that move vast amounts of air quietly or the squirrel cage type that use centrifugal force to drive the air channeled. Laptops, servers, and some desktop systems use fans of this type. They come in different CFM airflow and RPM rate of spin. I use these slot fans for adding airflow and keeping systems quiet.
http://www.firstpr.com.au/sys-admin/Silent-Hard-Drive/two-fans.jpg
