This is from the elders manual:
What kind of evidence is acceptable?
There must be two or three eyewitnesses, not just persons
repeating what they have heard; no action can be taken if
there is only one witness. (Deut. 19:15;Jol1ll 8:17)
Confession (admission of wrongdoing), either written or
oral, may be accepted as conclusive proof without other
corroborating evidence. (Josh. 7:19)
Strong circumstantial evidence, such as pregnancy or evi-
dence (testified to by at least two witnesses) that the
accused stayed all night in the same house with a person of
the opposite sex (or in the same house with a known
homosexual ) under improper circumstances, is acceptable.
The testimony of youths may be considered; it is up to
the elders to determine if the testimony has the ring of
truth.
The testimony of unbelievers may also be considered, but
it must be carefully weighed.
If there are two or three witnesses to the same kind of
wrongdoing but each one is witness to a separate incident,
their testimony can be considered.
Such evidence may be used to establish guilt, but it is
preferable to have two witnesses to the same occurrence
of wrongdoing.
This is clear. No accusation of one person will cause a DF or reproof, unless the party confesses.
If he denies an accusation from one person, there will possibly still be a Judicial Committee, counsel
will take place, then the charges will be dismissed. If the 2 elders investigating don't think he did it, or
are sure that no confession will take place, they often recommend no JC even be formed.
It's like- except for apostacy, the elders want to know that the person is guilty before a JC is formed, then
they can make him decide to be repentant or booted out. For apostacy, they hope the JC itself will cause the
person to confess, turn around and repent, or get out of the congregation.
There are actual cases of a fornicator confessing, but the party they fornicated with denying it took place.
So one gets reproof or DF, while the charges in the other case are dropped for lack of 2 witnesses.
Keep in mind that it is POSSIBLE for someone to lie in this case to get revenge, so the rules stand.