the ducts are connected and go to one outlet fan, its more efficient that way. The vents are there because there are no windows in he bathroom to open, and steam from showers, or smells need to be exhausted from the building.
To exhaust moist air caused when showering or bathing. If moist air is not removed it may cause condensation in the home or attic, which may result in mold.(Fan also gets rid of stink)