Of the some 70,000 disfellowshippings each year (some 1% of the membership) most were for sexual misconduct. Of these, I think I'm safe to say that very few were witnessed by a third party. That means that both participants had to voluntarily confess in order to satisfy their 2-witness rule.
Know of any that didn't meet this rule?
Let's add one more question to the mix -- say that one of them confesses but the other doesn't -- what does the judicial committee do then?