The Jews were expectin his return, and Elijah had never died a physical death, having been caught up in a whirlwind to God. Now the identity of the 2nd witness is where some disagreements come in. Some authorities see Enoch as the 2nd witness because he also had never died physically. A key to deciding who the 2nd witness is found by looking at Revelation and observing the actions of the two witnesses. The two witnesses have 4 distinct powers available to them.
Fire proceeds out of their mouth and devours their enemies.
The power to shut up the sky, so that it would not rain. The power to turn water into blood. To smite the earth with every plague. The first 2 abilities you will recognize as belonging to Elijah. And we see in Jude that Michael disputes with Satan over the body of Moses.