MASTER the Split-Screen EFFECT in Premiere Pro! (Here's HOW)
Tracks within the timeline are layer dependent, such that when images overlap, it’s the image residing on the track above the other that becomes visible, thereby obscuring the other image.
This presents a fundamental problem when zooming multiple images that each have an initial size smaller than full-screen size, which then in turn, temporarily become enlarged to full-screen size.
The solution demonstrated in this tutorial is to cut each video clip into smaller sections, and then reposition them in a vertical manner such that each clip takes its turn occupying the top track.