Apparently, it is "legal" (that is, no fine will be assessed) to be registered in two states so long as one does not cast a ballot in both states for the same election.
they should go further and examine who voted in both places in 2000.
Indeed, that is true. I was registered in one Pennsylvania county and filled out a form to change my registration from one county to the other. I was registered in the new one, but the old county was never informed of the change. I was registered in both counties even though I voted in one.