A double light switch is a receptacle with two switches mounted to the same electrical box. These are convenient when installed in bathrooms, because they allow you to control the exhaust fan with one switch and the lighting with the other. Separating the fan from the lighting enables you to save money because you do not have run the fan and the light at the same time if you only need one of them.
Turn the power off to the bathroom circuit in your circuit breaker panel. Unscrew the old light switch from the electrical box and disconnect all the wiring.
Run a two-wire electrical cable between the exhaust fan and the electrical box. Strip each end of the cable back approximately 1 inch to expose the individual wires. Strip these wires back 1/2 inch to expose the copper.
Connect the electrical wiring to your fan. Refer to your exhaust fan's instruction manual for wiring instructions and a wiring diagram. Fans vary slightly, so follow the manufacturer's instructions.
Go to the electrical box where the new switch will be located. Twist the white wire leading to the exhaust fan together with both the white wire leading to the lighting circuit and the white wire coming from the circuit breaker panel. Screw a wire nut on to these twisted wires.
Connect the black wire coming from your circuit breaker panel to the switch on the side that has the single terminal. Connect the black wire coming from your exhaust fan to one of the terminals on the other side of the switch. Connect the black wire coming from your lighting circuit to the other terminal on that side. These can be oriented to either terminal; the top terminal controls the top switch and the bottom terminal controls the bottom switch.
Twist the ground wires together, along with a short piece of ground wire. Screw a wire nut on to these twisted wires. Attach the other end of the ground piece of ground wire to the electrical box ground screw.
Screw the light switch to the electrical box and attach the cover plate.
Turn the power back on and test each switch. One will control the fan and the other will control the light.
Always run your bathroom exhaust fan when showering or otherwise creating a high humidity situation in the bathroom. This will reduce the chances of rot or mould setting in on your wood or drywall surfaces.