This a theory only as to how Enoch & Elijah might be the Two Witnesses if they already have immortal bodies that cannot die.

At the Rapture our bodies will be changed from mortal physical bodies that can die to immortal physical bodies that can't die. The question is will the Lord change Enoch's & Elijah's immortal bodies back to mortal bodies that can die so that they can be the Two Witnesses described in the book of Revelation 11 after which time they would die, be resurrected after 3 1/2 days and have their bodies changed back to immortal bodies again that can't die.