10/8/93 Awake article about child abuse misconceptions, checkout the second misconception: Children fantasize or lie about abuse.
This article notes how it's extremely unlikely that children lie in cases of abuse (95% in some estimates are truthful) and how even among the ones who lie it's usually when they make a claim (in truth) but then subsequently lie and say it didn't happen, when in actuality, it did.
Given this knowledge, why the two witness rule? 95% odds that the child who claims abuse is telling the truth by the Awakes own admission, that should be more than enough to change policies of the WTS.