MOSES / ELIJAH:
Most people think that it's these two, because of the nature of the miracles that are worked, and because of the visitation on the mount when Jesus was transfigured.
MOSES / ENOCH:
Some believe Moses is definate, not only due to the nature of the miracles, but also because of the mystery of Satan arguing over the dead body of Moses. Like, why would Satan even care about a lifeless corpse? They add Enoch because he was raptured, and didn't experience death.
NOTE: the key with Enoch is that he was "seventh from Adam". Raptured. As if this is a sign from God that at around the 7,000th (7 days) year of mankind is when the Rapture would occur. There are other passages, and maybe I'll create a thread on it one day.
ELIJAH / ENOCH:
Some believe this is the combination because neither Elijah nor Enoch tasted death. Both were raptured, and Heaven is not made up of flesh and blood. However, Scriptures also say that we are to be changed, and Elijah was clearly changed and reappeared with Moses when Jesus was transfigured.
So, those are the theories. But, I have my own ... which most have not been told or considered themselves.
First, I want you to know that I'm not so big on "tradition." Church traditions are often times, wrong. Furtunately, they do not effect Salvation. So, I don't promote tradition just because mommy and daddy believed it, or pastor so-n-so taught it. Keep in mind, Scriptures do not say who the two witnesses are, your Chruch tradition tries to suggest who they are. But, I know that none of these men worked miracles. Not a one of them. It is GOD who works miracles through His people. It is all Gods power. So, why would Moses be sent back into flesh just to die all over again? That doesn't make sense to me. Same for Elijah and Enoch. Is God so short of servants that he needs to send whom he already used? And why are they both from the Old Covenant instead of the New Covenant (ie - Peter or Paul)? Is this because the Dispensation of Grace is over? And so what? The Dispensation of the Law was long over before it. So neither apply.
My point is, it could be anyone. It could be the old prophets, or an apostle of the New Covenant, or anyone else. Again, since the power is of God, God can use whomever He will. Right, or wrong? Since the true Church is raptured, maybe God does send two souls back (since there will no longer be any real saints on earth post the Rapture).
Again, there are just my thoughts. Not asking any of you to adopt them. And for me, they are just thoughts, not doctrine. It could be Moses and me for all I know, lol. doesn't really matter. I'm just no pigeon-holed by a "tradition" that has been passed down and it ultimately the "only" option due to it being taught over and over again.