The two Witnesses are Moses and Elijah! 

They will prophesy from Jerusalem for 1,260 Days starting from First-fruit, the Day of the Rapture, on the 6th of April 2015 [Day 16 Month 1 Year 6000]!

They will start off their Ministry in their glorified bodies, however, because they'll not be returning to heaven for 1,260 Days, over time, their bodies will gradually change from being immortal to being mortal again! 

This is the reason why the Devil won't be able to kill them before they finish their Ministry!

They will end their prophesying on 18th of September 2018 [Day 8 Month 7 Year 6004], the Day they'll be killed! 

Then, 42 hours later [that is, three and a half days later], on the Day of Atonement, on the 20th of September 2018 [Day 10 Month 7 Year 6004], from Jerusalem, they'll rise from the dead and ascend up into heaven!

"And I will give power to my two witnesses, and they will prophesy one thousand two hundred and sixty days, clothed in sackcloth.” These are the two olive trees and the two lampstands standing before the God of the earth...When they finish their testimony, the beast that ascends out of the bottomless pit will make war against them, overcome them, and kill them." 

"And their dead bodies will lie in the street of the great city which spiritually is called Sodom and Egypt, where also our Lord was crucified. Then those from the peoples, tribes, tongues, and nations will see their dead bodies three-and-a-half days, and not allow their dead bodies to be put into graves." 

"And those who dwell on the earth will rejoice over them, make merry, and send gifts to one another, because these two prophets tormented those who dwell on the earth. Now after the three-and-a-half days the breath of life from God entered them, and they stood on their feet, and great fear fell on those who saw them." 

"And they heard a loud voice from heaven saying to them, “Come up here.” And they ascended to heaven in a cloud, and their enemies saw them." [Rev.11:3,4,7-12]