When one button operates two doorbells, the doorbells connect together in a parallel circuit. In a parallel circuit, multiple loads work independently of each other off a single electric source. If someone presses the button, power from the transformer's hot lead flows through the button to both doorbells at the same time. After activating the doorbells, the power returns to the transformer's neutral lead. The transformer's voltage rating must match the doorbells' voltage ratings. The transformer's amperage rating must meet or exceed the combined doorbell amperage ratings.
Turn off the electricity to the doorbell system's transformer. Often the circuit breaker that controls the building's lighting system also operates the doorbell transformer.
Run a section of an 18-gauge two-strand wire set from the transformer to the button and a section from each doorbell to the transformer. Cut each wire set section to size, using wire cutters. An 18-gauge two-strand wire set contains two insulated 18-gauge wires inside a plastic shield.
Remove 2 inches of the plastic shield from both ends of each wire set, using a utility knife. Use care to avoid nicking the wire strands' insulation with the utility knife.
Remove 3/8 inch of the insulation from each wire strand end, using a set of wire strippers.
Wrap the two wire strands at the button around the button's wire terminals, and tighten the terminals with the correct type of screwdriver. The button has two interchangeable wire terminals. One wire strand goes on each wire terminal.
Secure the wire strands to each of the doorbell's wire terminals, and tighten the terminal screws with the correct type of screwdriver. Both doorbells use two interchangeable wire terminals. One wire strand connects to each wire terminal.
Examine the transformer's terminal identification chart. The transformer identifies one low-voltage terminal as "Hot" and the other low-voltage terminal as "Common."
Wrap one of the button's wire strands around the transformer's "Hot" terminal. Tighten the transformer's terminal screw with the correct type of screwdriver.
Combine the button's second wire strand with one wire strand from both doorbells. Twist the three wire ends together with pliers and spin a wire nut over the ends of the wires.
Bend the two remaining doorbell wire strands around the transformer's "Common" wire terminal. Tighten the transformer's terminal screw with the screwdriver.