On Mon, 27 Oct 2008 11:52:30 -0700 (PDT), drhender
wrote:

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?

There is not an easy way, but there is a straight forward way.

Identify all the devices on the two breakers. May as well start with
the lamp you are working on. It should have an in and out. When you
break it at that point, half of the stuff should go out with one
breaker.

You should work it back until you get to the point where the two are
joined.

If you want to save yourself a headache, you should take a picture of
the box before you break any taps.