ANSWER: The first question we will briefly tackle is WHY must there be TWO witnesses, and not one? Why not four, five, six or even more? God's law insisted it was necessary to have, at the minimum, at least two witnesses to make good an accusation against a person (see Deuteronomy 17:6, 19:15). This principle is reiterated by the apostle Paul to his friend Timothy (1Timothy 5:19). The opinion of one person was not enough to convict someone of a sin or crime. The testimony of at least two people were needed in order to condemn a person to death (Hebrews 10:28). One of the primary purposes of these two individuals is to witness against the end time Beast and False Prophet, who will ultimately receive the death penalty from God for their sins (see Revelation 19:20).
The two witnesses will be "called or commissioned" to prophesy for 3 and 1/2 years. The word "prophesy" in biblical terms means to foretell divine events or to speak under the inspiration of God. Part of their message during their ministry will include preaching, holding up, and maintaining the truth of God before the world. They will give warnings and uttering judgments against those who are not following God. They will call people to repent of their sins, and to fear and glorify God. They will testify that Jesus is the Son of God, the heir of all things, and the coming King of Kings. Lastly, they will adminster encouragement to the remnant of God living on the earth at that time.
Many, many people throughout the years have speculated who might be the two witnesses. While the exact identity of these two individuals still remains a mystery, one thing that can be asserted is that they will be literal, flesh and blood people just like the Beast and the False Prophet. They will not be angels.
There is not a single account in the Bible of human beings hurting angelic beings. Although no one will be able to stop or harm God's witnesses during their 1,260 day ministry (Revelation 11:3), after it is completed the Beast will be allowed to kill them (verse 7). Their dead bodies (angels do not have bodies of flesh) will then be allowed to lie in Jerusalem, unburied, for three and one half days until they are finally resurrected. God uses human beings to do His work on this earth, as humans are destined to become His Sons. Angels are merely God's messengers (Hebrews 1:13 - 14).
Ultimately, God will reveal the true identity of his two witnesses when, in his wisdom, he determines it is the right time to do so.