Who are the two witnesses of Revelation 11?

They are the two "olive branches" (Zechariah 4:12), witnesses sent for the sake of the Jews and the world after the Church has been removed. God did not ordain two avenues of salvation to take place on earth simultaneously, so the Church cannot be here during that time. The Old Testament ordination ended with Jesus' crucifixion, which is clearly illustrated by the abolition of the Temple service the moment Jesus died when the curtain in the Holy of Holies was torn from top to bottom.

The two witnesses appear before the seventh angel sounds the trumpet of judgment. The world at that time will be in a terrible shape due to the Antichrist's deception. Revelation 9:20-21 says: "And the rest of the men which were not killed by these plagues yet repented not of the works of their hands, that they should not worship devils, and idols of gold, and silver, and brass, and stone, and of wood: which neither can see, nor hear, nor walk: Neither repented they of their murders, nor of their sorceries, nor of their fornication, nor of their thefts." In other words, the world will be given over to idolatry. The Church will be gone, and with it, the light will be gone as well. Darkness will prevail as the world follows the system of Antichrist. But because God never leaves the world without a testimony, these two witnesses appear with a message that exposes the Antichrist's lies.