To wire a double switch youll need to cut the power remove the old switch then feed and connect the wires into the double switch fixture. Sure the fan is great to have on during a shower but its unnecessary when brushing your teeth.

Multiway Switching Wikipedia
Wiring a bathroom fan and light to one switch uk. For example you might want to turn on a bathroom light separately from the ceiling fan. If you have one of these you may have fastened the fan and light to separate the switches. Most bathrooms however add at least one other light fixture often a wall mounted vanity light above the mirror or a pair of wall sconces on the sides of the mirror. By wiring the two separately you can control each independently resulting in a more efficient bathroom. Now youd like to operate both of them from the same switch. Bathroom exhaust fans with built in light fixtures are fairly common.
This can be a combination vent fanlight fixture unit. Instead we are going to install a new switch that will turn teh fan on and off. This wiring diagram shows an easy to follow configuration for a bathroom extractor fan fitted with a timer thats not going to be turned on by the existing light switch. Only having one switch to power more than one fixture is never a problem as long as you do not exceed the amperage for which the switch is rated. And one easy way to waste electricity is by having a bathroom fan and light that do not work independently. Hooking up a bathroom fan and light on the same switch is actually a more convenient way to wire two fixtures than adding a separate switch.
The wires are already where you need them to be so you dont have to run new ones. If you have one of these you may have wired the fan and light to separate switches. The nec requires at least one ceiling mounted light fixture in a bathroom. Though it is not difficult to wire a double switch careful attention to safety is crucial to prevent injury. How to wire a bathroom fan and light on one switch bathroom exhaust fans with built in light fixtures are pretty common. Now they would like to operate both of the same switch.