The two witness requirement only comes into play in deciding how the congregation will handle the matter. It has nothing to do with whether the police are informed. The authorities are informed if the law requires it, or if the victim, parents or anyone else wishes to make a report. I hardly think that's unique to Jehovah's Witnesses. Other churches, professional bodies and even child protection agencies have similar policies.
The two-witness requirement is based on the Bible; that is how it says accusations should be handled. It is well known that in most other churches, they won't disfellowship you whether you have two, three or twenty witnesses. So it's a bit inconsistent to single out Jehovah's Witnesses for criticism because of the requirement for two witnesses for congregational action.
You can criticize the two-witness requirement, but the alternative would be to disfellowship everyone accused of child molestation, even when there is only one witness. When it's one person's word against another, elders have no way of knowing who is telling the truth. That's not their fault; it's just a fact of life.
It's nothing to do with telling the victim that you don't believe them. It's not what the elders believe that counts, it's what can be proved. It's just the way that cases have to be handled. The elders may well feel that the victim is telling the truth. But just as a judge can't convict based on a feeling or a hunch, elders can't disfellowship someone without satisfactory evidence. And according to the Bible, that means two witnesses. The Bible says: "One witness may not testify against a soul for him to die." (Numbers 35:30)
Naturally, the two witnesses don't have to be eyewitnesses of the same event. Two complaints by separate victims are sufficient for a judicial committee to be set up. And since a report of all allegations goes to Bethel, these could even be in different congregations and at different times.
Elders will report the abuse allegations even if there is only one witness, if that is what the law requires. And you aren't disfellowshipped for reporting an abuser to the police. A 2002 letter to the elders made it quite clear:
"Child abuse is a crime. Never suggest to anyone that they should not report an allegation of child abuse to the police or other authorities. If you are asked, make it clear that whether to report the matter to the authorities or not is a personal decision for each individual to make and that there are no congregation sanctions for either decision. That is, no elder will criticize anyone who reports such an allegation to the authorities."
No-one has been disfellowshipped for reporting child abuse to the police. Even the Silentlambs site doesn't mention a single case. That is just a fiction invented by opposers. In fact, a previous poster has proved it to be false, acknowledging that the parents of a molested child went to the police and as a result the abuser went to jail. Clearly, they weren't disfellowshipped for what they did; a year later they were still attending the book study.