Recently, I was replacing a light fixture in my home. When I
attempted to cut the breaker to that fixture, I discovered that I had
to turn off two breakers to kill the circuit that includes the light
fixture. My assumption then, is that the two circuits (one for each
breaker) that should be separate have somehow been crossed together
such that they both breakers effectively power a single circuit.

I'd like to fix the problem. Is there a simple way to determine where
the two circuits have been connected?