don't you end up with two essentially different programs that test and execute differently?

You end up with two programs that are functionally equivalent. If their output diverge, there's a bug in at least one of them. Or in the program that compiles the Word document, as it happened a couple of times.

Is that why there was that "low level" radioactive material leak in a Canadian reactor? ;)

This? It sounds so minor! It would probably be worth it for the excitement of getting something other than zero on the periodic urine tests used to detect alpha and beta doses. Badges detect gamma.