Reference to the "Two Witnesses," or two [endtime] prophets, can be found in Revelation 11:3-12. They have been compared with Elijah and Moses, in the Old Testament. Whoever they may turn out to be, they will certainly possess amazing powers, with which to certify their authority. The Revelation says that these two men will prophesy to the world for the final 1,260 days (three and a half years, or 42 months) of the final seven years before Jesus returns.
Because it is common for mentally unstable individuals to claim to be one of the "witnesses", most churches have shied away from any mention of the Two Witnesses at all. But the genuine should not need to give place to the counterfeit. With or without mental patients, there will be two endtime prophets declaring the truth to the world.
It is significant, however, that there are Two Witnesses, and that most pretenders to the role of endtime prophet operate independently... because their own delusions of grandeur make it impossible for them to submit to the counsel of others.
There is a biblical principle of everything spiritual being "confirmed in the mouth of two or three witnesses". (Matthew 18:16, II Corinthians 13:1, I Timothy 5:19, and Hebrews 10:29)
If it should happen that someone must act entirely independently, however, then it seems that clear evidence of supernatural power may be regarded as a second "witness". (John 5:36)