Not to be in disagreement with your summation that the 2nd witness will be Moses, but the scripture you quoted has a parallel in the Gospel of Mark that states that Peter "didn't know what to say" so he said that, not that he didn't know what he was saying.  And since the Gospel of Mark was supposedly written by Peter's nephew, I think it more likely that was the case.  Not to mention that Peter was known for making rash statements without thinking them through.  We have him being praised by Jesus in one verse and two verses later, Jesus is chastising him for the toejamitis (or foot-in-mouth disease).

Mark 9:5-6 (KJV)

9:5  And Peter answered and said to Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias.

9:6  For he wist not what to say; for they were sore afraid.

Here are some more translations of Mark 9:6:

NKJV - because he did not know what to say, for they were greatly afraid.

NLT - He didn't really know what to say, for they were all terribly afraid.

NIV - (He did not know what to say, they were so frightened.)

NASB - For he did not know what to answer; for they became terrified.

The interesting thing is that if they were filled with the Holy Spirit and prophesying (which the Holy Spirit is necessary to do) then they should not have been afraid.  This just happened to be one of those toejamitis moments in the gospels by Peter.  It was after this statement that Jesus told them to "NOT" tell anyone until after the Cross.  The reason for the Tabernacles were to honor what was seen and heard there ( i.e. testify of what they witnessed).  Jesus tells them NOT to do so until after he died (yet another rebuke for Peter's problematic foot-in-mouth disease).