It turns out that it's possible to "spawn" two different Zeldas during the Tower Collapse sequence, and even lead the second Zelda into a trigger to unlock one of the gates. The way it works is pretty simple.
The scene that covers exterior portion of the tower actually spawns Zelda four times. Each one checks the spawn id variable (used to identify which spawn Link used to enter the scene), and unloads itself if the id doesn't match up with the one it is suppose to be bound to. From top to bottom, the following is a list of entrance indexes that will spawn one of the Zelda's:
- 0x043F, spawn id 00 (Death Cutscene, from Ganondorf Boss Room)
- 0x032C, spawn id 02 (From Tower Collapse Interior, Boss Room Floor (Lower))
- 0x0334, spawn id 04 (From Tower Collapse Interior, Burning Rocks (Lower))
- 0x051C, spawn id 06 (From Tower Collapse Interior, Stalfos Fight (Lower))
In other words, if Link enters with a base entrance index of 0x032C, 0x0334, or 0x051C, and the game warps him up to the top to play the cutscene, two Zeldas will remain spawned. In the video above, it looks as if a base index of 0x032C was used.