This is a very old belief. Conjecture it is, however there are no other figures to fit the prophecy.
Ok, first, the fact that they never tasted death as we know it is not a condition that they should have to come back to the earth and die. Yes, Hebrews 9:27 says, "And as it is appointed unto men once to die, but after this the judgment" but to take that verse totally out of context and apply it to Enoch and Elijah is wrong. They were of two different dispensations, two different ways of doing things, two different covenants, pre-Flood and post-Flood, and we are in a third dispensation, the Dispensation of Grace. You can't take a verse like that in Hebrews and try and make it apply to Genesis or Kings.
Second, why do we have to try and put them into the equation? Why do we have to figure out who these two individuals will be? The Bible says "two witnesses." They are in the Old Testament described as "two olive trees, two olive branches, two anointed ones." That's it; no earthly names or designation are given to them. That's good enough for me. But to say that God will bring Elijah, Enoch, Moses or anyone else from Paradise back to earth for 3.5 years, there's no Biblical truth to it.
Now it’s you who is conjecturing. You do not know they are in Paradise, they never died. To engage in conjecture myself, they could have been transported across time for all we know.
Rigid dispensationalism does a disservice, if you have broad statements of scripture, regardless of time, place, covenant or Testament, it applies broadly. Are you seriously claiming that it is no longer appointed for men once to die?