2 Thess. 2:3 tells us that there will be a "falling away" first, and the man of sin, the son of perdition will be revealed. If Satan & his angels were cast out of heaven first as the "falling away," it would have to be at the beginning of the 1,260 days Great Tribulation period when the Antichrist has authority for 1,260 days or 42 months when he reveals himself.

However, the Two Witnesses are killed by the beast (Antichrist) between the 2nd & 3rd woe with the 3rd woe (Rev. 12:12) being Satan who is cast out of heaven down to earth with his angels.   The 3rd woe happens after the 7th trumpet is sounded which is not at the beginning of the Great Tribulation period, but it happens before the 7 vials or bowls of wrath are poured out on the Earth during which time Satan would be then present on Earth before the end of the 1,260 day Great Tribulation period.