Duplex receptacles can be installed side-by-side in a double-gang electrical box and connected to the same or different circuits. This is called a double gang outlet, or a quad outlet. When you run both of them on the same circuit, the easiest way to wire them is to use short lengths of wire to connect them together. Outlets are designed to make connecting them in this way easy. If you do this, plan the circuit so it will not be overloaded by the appliances you plan to plug in to the pair of outlets.
Safety First
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Turn off the power to the circuits you are working on by flipping off the circuit breakers. Even after doing this, check the wires with a voltage tester before you work on them. Circuits are often mixed up, and the breaker you turned off might not be the one that controls the outlet. If it turns out you turned off the right breaker, you lose nothing by double checking, but you save yourself from a painful shock if it turns out you turned off the wrong one.
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Daisy Chaining Is Not Wiring Outlets in Series
When you make a double gang outlet by daisy chaining them, you're actually wiring the outlets in parallel. It's a small difference in terminology that makes a big difference in practice. If you actually wired the outlets in series, you would have to plug devices into the both receptacles on the first outlet, and the devices would have to be working for the second outlet to get power. Christmas lights are wired in series, and everyone knows what happens when one of the bulbs burn out. The entire string goes off.
Procedure for Wiring a Double Outlet in a Box
Start by wiring the first receptacle to the live circuit cable. Pull the cable through a hole in the back of the electrical box, separate the wires and strip an inch of insulation off the end of each one. Attach the black wire to the bottom brass lug on the outlet. Connect the white wire to the bottom silver lug in the same way. Finally, make a hook in the end of the bare ground wire, wrap it around the green lug on the bottom of the outlet and tighten the screw.
if you are wiring the outlets on different circuits, you'll have a second live cable in the box. Connect the second outlet to that cable using the same procedure, and you're done.
If you want to wire them to the same circuit, you'll need jumper wires. Remove the individual wires from a 6-inch length of cable of the same gauge as the wires in the box and expose the ends with a wire splicer. Connect one end of the black wire to the top brass lug on the first outlet and the other to either brass lug on the second. Connect the white wire to the chrome lugs in the same way and the ground wire to both of the ground lugs.
Finish up by attaching both outlets to the electrical box with the screws that are provided with the outlets and screwing on a four-hole outlet cover.