this was never clear to me before but when you compare the chronology in matt 24 to the 6th seal events, it is clear that they witness during the 2nd half, when God is pouring out his wrath on "those that dwell on the earth"
Gods wrath does not effect his saints, only
"those that dwell on the earth" (this always refers to unbelievers)
it is the beast that kills them, so Gods wrath has nothing to do with they're tribulation
To start, we know that the abomination of desolation takes place at the midpoint of the Seventieth Week. And we know that the sixth seal takes place after the midpoint of the Week because we see the same triple sign in the sun, moon and stars that Jesus said would take place after the abomination that causes desolation (Rev. 6:12-13, Matt. 24:29). And we know that the trumpet judgments can only begin after the seventh seal is opened.
Now this finally brings us to the chapter in question—chapter eleven. Verses one through fourteen tell us what the Two Witnesses will do and how long they will do it. They will prophesy for 1260 days (Rev 11:3). After their work is completed they will be killed (11:7), they will lay in the street for 3.5 days (11:9), and then they will be brought to life and will be raised to heaven (11:11-12). In that hour there will be an earthquake with thousands of people killed (11:13).
The second woe is past; behold, the third woe is coming quickly. (Rev. 11:14)
The second woe is said to be in the past once the ministry, death and resurrection of the Two Witnesses is completed, and because the second woe is said to be associated with the sixth trumpet it is clear that their death takes place during the period of the sixth trumpet, and that the ministry of the two take place in the last 3.5 years because, as we discussed in the second paragraph, the seals fill up the first half of the Week and continue into the second half only to lead into the trumpet judgments. And in nice, orderly fashion, the very next thing mentioned in Rev. 11:15 is that “the seventh angel sounded…