depends on the local code and size of the vent.
I know Nutone makes a multiple bathroom with one fan and vent system. I also
know on the only installation that I saw the inspector required back draft
dampers on all the runs after the motor.
Most new vents fans enclosers have built in dampers to prevent backflow, If
not install and external one then will be OK.
Also they make a system that has an inline fan unit in the attic that all
vents run to, no noise with those but are pulling air from all vents when
only one is needed.
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