I just learned how to do 3-way wiring, so that I can turn a light off from 2 different switches. My question is, how do I add another light to that mix, so that both lights come on/go off at the same time when I turn the switches on and off? (It's for a closet that has 2 lights in it that I want to come on/go off at the same time).
Very basically, you must have two wires going from light to light. A black wire and a white wire.( A bare copper wire should be included with the pair. It is a ground wire and grounds both light boxes together.) Back to the black and white wires. The black wire should be connected to the black switch leg wire at the first light box as well as to the first light fixture. The white wire is the neutral and should be connected to the white wire of both light fixtures. That's called a parallel connection. Just a note: There are several ways to wire three way switches, so my answer presumes there are two wires going from one of the switches directly to the light fixture. (A white wire and a black wire).
You have to tie the wires that go to the light together. It is called parallel.