Regarding the two witnesses:

Revelation 11:3 And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth.

What are they witnesses of?

Are they, like the apostles, witnesses of the resurrected Jesus?

Acts 1:21 Wherefore of these men which have companied with us all the time that the Lord Jesus went in and out among us,

  22 Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection.

I Corinthians 15:8 And last of all he was seen of me also, as of one born out of due time.

If not that, then what are these two witnesses of?