A double gang light switch is designed to fit in a single-gang switch box, yet it offers the capability of controlling two separate circuits. The switches are situated one on top of the other, and instead of the traditional switch plate, an outlet plate is used to finish the installation. In order to operate two separate lights, there should be at least one feed in the box and two traveller circuits (one for each light).
Turn off the breaker that powers the feed line in the wall box. Always double check the line with a voltage tester to ensure that you've turned off the correct breaker.
Use the wire strippers to remove about 3/4 inch of insulation from each insulated wire in the box.
Connect the three white wires together with a wire connector, wrap it with electrical tape and stuff it back into the box.
Connect the three ground wires together along with a pigtail with a wire connector, wrap it with electrical tape and stuff it back into the box. Connect the pigtail to the ground screw on the double gang switch.
Look at the terminals on the double gang switch. On one side you should see two screws connected by a small piece of metal. On the other side you will see two screws with no metal bridge.
Connect the feed line to one of the screw terminals on the side that has the metal bridge. This is done so both light circuits will receive power from the same source. Tighten down the second screw on that side.
On the side of the switch without the metal bridge, connect the remaining black wires, one to each screw terminal.
Wrap the terminals with electrical tape and carefully push the switch into the box. Secure the switch to the box with the top and bottom screws.
Install the cover plate and turn the circuit back on.
Test your switch to make sure it works properly by turning each switch on and off.
Never work on an electrical circuit while it is hot. If you have any reservations about working with electricity, even when it's off, please call a licensed electrician to perform the job.