I think it might be the same two witnesses that God sent to witness against Sodom and Gomorrah. They were two angels who were disguised to look like two men. The powers it describes the two as having are also powers that are attributed to angels. And the End Times are described as being as it was in the days of Lot. God's judgment against Sodom was an early foreshadowing of God's final judgment on the world at the end of time. So I think it's reasonable to speculate that God might do it in a similar way again using the same two witnesses. This interpretation isn't in the commentary of either of the two types of bibles I own (RSV-CE and NAB). But it makes sense to me. Anyone have any thoughts about it?
P.S. - I do not claim to know when the Second Coming of Christ will be.