The purpose of this Law is to show the truth behind a doctrine in a “proving manner”.
Here is the verse from which this explanation of the “Law of Two Witnesses†is based.
This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.
2 Corinthians 13:1
There must be at least two verses to affirm a doctrine (a teaching). Two which affirm the above verse would be Deuteronomy 17:6 or 19:15.
At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.
Deuteronomy 17:6
One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.
Deuteronomy 19:15
The purpose is to show the two witnesses are necessary to prove something to be true or false. In this case a teaching from the Bible.