Some arguments for the Pre-Trib Rapture being a separate event to the Second Advent are:
There were 2 phases to the First Coming.
The church is not appointed to (God’s) wrath.
The church is not mentioned between Rev 3 and Rev 22:16.
The “blessed hope” versus the “Time of Jacob’s trouble”.
Who populates the Millennial earth?
There is a known day (Rapture) and an unknown day(2nd Advent), the righteous removed (Rapture) versus wicked removed (2nd Advent), the Lord himself (Rapture) versus “with the angels”.