The great multitude appearing in heaven is closely related to the seventh seal being opened. The great multitude appear in heaven when the rapture takes place.
For those who believe that the trumpets follow sequentially after the seals, the rapture is thought to occur pre-tribulation when the sixth or seventh seal is opened (the trumpets being God’s wrath). These brothers and sisters then have to explain away the verses that link the rapture to the seventh trumpet. The first six seals certainly sound like tribulation too!
For those who believe in sequential seals and trumpets, and believe the rapture is at the last trumpet (post tribulation), they have to explain away the fact that the great multitude appearing in heaven is closely linked to the sixth and seventh seals.
If the Seventh trumpet is concurrent with the seventh seal, then there is no discrepancy. The Church is raptured in the twinkling of an eye as soon as the seventh trumpet begins to sound (Rev 10:7). The rapture is post-tribulation (after 1260 days) but pre-wrath (before the bowls of God’s wrath are poured out during the seventh trumpet).
The Church will be spared God’s wrath (1Thes 5:9) which is poured out when the seventh trumpet sounds. (Rev 11:18)
The Church will be raptured at the last trumpet as soon as it sounds (Rev 10:7 & 1Cor 15:51-52).
Jerusalem will be trampled for 42 months (Revelations 11:2) while the two prophets prophesy for 1260 days (Rev 11:3).
Israel will be protected for 1260 days; times time and time and a half. (Rev 12:6,14) while the dragon pursues her offspring (the Church). (Rev 12:17)
The beast is given authority for 42 months to make war against God’s holy people and conquer them. (Rev 13:5-7)
Then the seventh trumpet sounds (Rev 11:15) and the rapture and the first resurrection take place. (Rev 10:7 & 1Cor 15:51-52)