Watchtower issues are a forest, not a tree. The more people barking up individual trees the better, and the two witness rule is definitely a issue worth exposure.
There are often comments requesting the one question to help a JW leave, or the main issue to expose to bring the religion down. There is no silver bullet. JWs have a vast range of reasons for why they think it is the truth, or why they want to be part of the religion. It takes a variety of tailored approaches to help, and exposure of each and every issue is worth the effort.