Well, my favorite author whose style I prefer to study is Robert Jordan author of the Wheel Of Time series. He is known for shifting the POV between characters in the same scene. He wrote the series in 3rd person omni, and I never had a problem with the POV shifting in terms of finding it jarring or such.
For reference, I didn't have to look far as I went back through chapter 1 of book 1 (Eye of the World), and within the first chapter we see the POV begin with Rand and shift to his father. They are in the same scene when this happens walking side by side.
I'd like to think the rule is a guard rail guideline. It keeps the writer from spilling over onto the freeway of confusing the reader if they are careless.
In the end, it's a gray area for me, and I would depend on reviewers to opine if they were confused or not when it changes in the same scene.