Split screen
Watch as an animation →Show two scenes on screen at the same time to emphasize comparison or simultaneity.
Before/after, a phone call between two people, different places at the same time.
In 9:16, a top-and-bottom split (crop, then vstack) feels natural. Take the cropped-out areas into account and compose the start frame so the subject sits within the central half of the frame.
Stack two 9:16 clips into a top-and-bottom split screen, each cropped to its central half: ffmpeg -i top.mp4 -i bottom.mp4 -filter_complex "[0:v]crop=1080:960[t];[1:v]crop=1080:960[b];[t][b]vstack" -an out.mp4. Keep both subjects in the middle of their frames so nothing important is cropped.
Not verified