Hi Cliff,
 
I did a word search on "Sackcloth", "two witnesses", and "Prophesy".  Here is what I determined to be the main meanings of these words in the Bible:
 
Sackcloth.....Sackcloth was put of when people were mourning.
 
Two witnesses....it appeared to me that most of the time two witnesses were used, it was pertaining to accusing some one or warning of coming judgement.
 
Prophesy....is the "foretelling" of something.
 
Again this is my idea of what the words were used for in scripture.  So, It seems probable to me that the two witnesses will be witnessing that the world is deserving of the judgement of God's wrath that is soon coming, and prophysing (warning) the people on earth that it is coming.  They will be wearing sackcloth because it signifies mourning for the trouble that is coming on the earth.
 
I am not being dogmatic in this, as I have always thought/heard that they were trying to bring souls into the kingdom, but now I think that may not be their job or purpose.
 
I think this could happen in the last 3 1/2 years because Rev 11:7 says,: "And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them and kill them."
 
That verse doesn't really say "WHEN" the beast came out of the pit, only saying where the beast came from.
 
 
Donna