"Those from the peoples, tribes, tongues, and nations will see their dead bodies." This really confused commentators up until about 1940 or 1950, because they did not understand how the whole world would be able to see dead bodies in Jerusalem. So, scratching their heads, they thought of it as a mystery. Now we know: Television makes it possible for us to see anything that happens anywhere on earth. With the modern technological advancements over the last half century or so, we have a better grasp on how Bible prophecy will be fulfilled.
The next matter is the phrase "not allow." They do not allow the Two Witnesses' dead bodies to be put into graves, which is the ultimate in disrespect and desecration. Amos 2:1 shows how much God is against this sort of thing, which is why this detail is mentioned in Revelation 11. Amos is summarizing God's judgments on the nations surrounding Israel and Judah:
Thus says the LORD: "For three transgressions of Moab, and for four, I will not turn away its punishment [meaning, I will punish], because he [the king of Moab] burned the bones of the king of Edom to lime." (Amos 2:1)
The Moabite king evidently exhumed the dead body. Then, in the sight of the defeated Edomites, he burned the bones of one of their kings. To defile the dead is so heinous a crime that God says, "Just for that, I'm going to come and wipe you out." It was a terrible thing for a nation to perpetrate against the king of another nation—despite his being dead for perhaps many years. God is very much against this sort of desecration.
Richard T. Ritenbaugh