They don't have to start at the same time but they do have to play together simultaneously & using one sound for each ear would just cause imbalanced sound distribution. I would also have two idle "brain core" if I'm not watching anything (see my following comment for context).
Ah, I understand now what you are getting at.
Apparently, me & my new parents (at the time) was told by a psychologist during my adoption 9 years ago that I can process various different information simultaneously. To put it in CPU analogy, my brain as a processor has 4 cores each processing different unrelated task at the same time without disrupting the other process. So for me, watching all that YouTube video that you linked, filtering them each into different process in my mind without losing focus on one or more video is possible for me, like I am doing right now which is watching two different TV series & listening to a music all while typing this.
I suppose this is what you meant with your question?