In the last verses of the Old Testament,
God makes a definite promise the He will
send Elijah back to Israel just before the
Day of the Lord.

If Elijah isn't one of the Two Witnesses,
we would have to find someone else who
would fulfill the description better. This
would be difficult to do.

Elijah must be one of the Two Witnesses,
in order for the prophecy in Malachi can
be fulilled.