The Nephites, even those in Ammonihah, were aware of the two-witness rule. Deuteronomy 19:15 says:
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. (see also Deuteronomy 17:6; Numbers 35:30; 1 Kings 21:10)
That becomes a significant principle in what happens in Ammonihah. In The Legal Cases in the Book of Mormon, I explain the implications of this complaint as follows:
The people of Ammonihah rejected Alma’s testimony out of arrogance and incredulity, to be sure, but their rejection also had legal grounds. Their penchant for legal detail manifests itself when they reject Alma’s testimony on the technicality that he appeared to be a sole witness or testifier. Rather than addressing the truthfulness of Alma’s claims by accusing him of being a false witness or a false prophet (as had been the failed strategy of King Noah and his priests against Abinadi), these people argued that if God were to condemn this city as an apostate city, he would need more than one witness to stand against it in such a weighty matter: "Who art thou? Suppose ye that we shall believe the testimony of one man, although he should preach unto us that the earth should pass away? … Who is God, that sendeth no more authority than one man among this people?" (Alma 9:2, 6). An accusation such as this one for apostasy, they correctly and forcefully argued, needed to be supported by two witnesses.
In our legal system, we do not require two witnesses. In Biblical and Jewish law, they had to have two witnesses. How is the principle of two witnesses exercised among Latter-day Saints today? Missionaries and ministering sisters and brothers travel two-by-two. At baptism, there must be two witnesses. Similarly, in bestowing the gift of the Holy Ghost, and in the marriage sealing ordinance, and so on. Even the notion of the presidency of the bishop and two counselors is based on the principle that in the mouth of two or three witnesses shall all things be established. You cannot, or at least should not, operate as a presidency if you do not have the concurrence of your counselors.
We use this principle more often than we appreciate, and it is an important one. When you are involved as a married couple, within a missionary companionship, or as a presidency, learn to use the power of the two-or-three-witnesses principle. There is real wisdom and strength here.
John W. Welch, "The Trial of Alma and Amulek," in The Legal Cases in the Book of Mormon (Provo, UT: BYU Press, 2008), 237–271, quote on 242.
Robert L. Marrot, "Witnesses, Law of," Encyclopedia of Mormonism, 4 vols., ed. Daniel H. Ludlow (New York, NY: Macmillan, 1992), 4:1569–1570.