Some think the Two Witnesses have the spirit of David, and that there are two of them. That is, the spirit of David (father) and the spirit of Christ (his son). As to how true this is, we would have to read Revelation 11:5-6 in context.
The description found there best suits the spirit of Moses and Elijah, who did the signs and wonders stated in the Scripture (cp. Matthew 17:2-3). And moreover, this also agrees with and is consistent with the Scripture of prophecy concerning the end times (see Malachi 4:5; cp. Matthew 17:11-12).