The conclusion of that study was that parallel development mainly accomplishes a false sense of security, and most of the additional reliability in those projects came from other sound engineering techniques. But I have lost the reference, so I don't know how much credibility to lend my memory.
Leveson also wrote the report on Therac 25.
(Also to protect against physical failures it works, because physical failures are more independent than software ones, as far as I understand.)
