We know that these two witnesses are described as being "two olive trees, and the two candlesticks standing before the God of the earth" (v. 4). [I am not sure that the word "God" should be capitalized; Satan is called "the god of this world" (2 Cor. 4:4), and it is the re-animated Antichrist, indwelt by Satan, before whom they defiantly stand (Rev. 13:3-4).] The New Testament uses the olive tree to describe the Kingdom of God, more specifically, the Church. Paul tells us that there are two "branches" that comprise the Church: the Jews, or natural branches, and the Gentiles, the wild branches (Rom. 11:17-36). From this, I believe that one witness is a "former Gentile," and the other is a "former Jew."
This is supported by the two candlesticks, which must be interpreted in the context of what they represent in the Book of Revelation. In his vision, John tells us that a candlestick represents the light radiated by a church (Rev. 1:12-13, 20; 2:1). We know the Church is made up of "the two loaves of the Feast of Pentecost"; the loaves represent the converted Jews and the converted Gentiles, which were made with leaven, leaven being symbolic of sin (Lev. 23:17-23; Acts 2:1-47). For all those believers and unbelievers reading this, if you don't already know this, Christians still sin. That is why we have 1 John 1:8-10!
So, based upon the fact that "it is appointed unto men once to die" (Heb. 9:27), the two witness must be Enoch, a Gentile, and Elijah, a Jew. That is because neither Enoch, nor Elijah died (Gen. 5:24; 2 Kgs. 2:11).
I cannot prove this, but in order for Hebrews 9:27 to be true, it makes perfect sense.