Is it possible to vent two exhaust fans through a single wall penetration?
I don't know about code compliance.  But, this might work for you:

1 Y-Connector at the outside vent.



2 Backdraft Dampers (one on each leg) at the Y to stop the backflow of noxious gases from one fan(on) to the other (off).

  

Note that the backdraft dampers will add to the resistance of the duct work.  You may wish to move up to the next size of duct.