When Jesus said to James and John, "Ye know not what manner of spirit ye are of":

Luke 9:54 And when his disciples James and John saw this, they said, Lord, wilt thou that we command fire to come down from heaven, and consume them, even as Elias did?

  55 But he turned, and rebuked them, and said, Ye know not what manner of spirit ye are of.

He wasn't saying that the Spirit of God wouldn't do that?

Because, didn't Elijah, by the Spirit, do that very thing?

II Kings 1:10 And Elijah answered and said to the captain of fifty, If I be a man of God, then let fire come down from heaven, and consume thee and thy fifty. And there came down fire from heaven, and consumed him and his fifty.

Yes, one situation is in the old testament and the other in the new testament,

however, the character of the LORD didn't change between the two testaments,

but doesn't he work differently in the two testaments?

Yet it is the same LORD, could that in any way be related to:

Isaiah 11:2 And the spirit of the LORD shall rest upon him, the spirit of wisdom and understanding, the spirit of counsel and might, the spirit of knowledge and of the fear of the LORD;

In Isaiah 11:2, wasn't there seven characteristics of the Spirit listed?

Is that similar to:

Revelation 3:1 And unto the angel of the church in Sardis write; These things saith he that hath the seven Spirits of God, and the seven stars; I know thy works, that thou hast a name that thou livest, and art dead.

So that the same Spirit could work one way with Elijah in the old testament, and yet another way with James and John in the new testament?

So, would Luke 9:55, preclude John from being one of the two witnesses?

Revelation 11:6 These have power to shut heaven, that it rain not in the days of their prophecy: and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will.