As Eden already correctly wrote, the 2-witnesses-rule is wrong, when it is used as a reason for not having to report to the police a criminal offense of which you have been informed.
If a "brother" confesses he has murdered a 10 year old kid to a JW elder, or if someone reports that the has seen a brother murdering a kid, then i believe the elder would not need a 2nd witness to report this to the police.
Not reporting what you know about such a crime, exposes the congregation members and the society at large to the crime to be repeated.