When my son was about 4 or 5 he used to draw pictures with pens in both hands simultaneously which I was always amazed he could do!

And you can subvocalize (that is, think them in your mind) two different voices at once if you deliberately try to, and it's a skill that gets easier with practice. Though, no matter how much I practiced it, I was never able to get to where it would take place automatically without my forcing it.

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Left ear: Mozart, right ear: The Bible in Chinese

Async-programming style single-core concurrency would still look like parallelism to an outside observer :)

That's not a good argument. First, music is not speech. It doesn't require linguistic processing. Second, you can hear chords instead of individual voices, so polyphony can be perceived as shifting harmony, and it usually is.

Not reading out loud, but I've caught myself a few times on reading and not processing that, because I was thinking about something else. Like I still did the reading, but straight to /dev/null of my brain

I experienced this too, when I started reading out loud more. At first, it was just that my eyes would scan ahead a bit from what I was saying, to help me get the right emphasis by knowing where the sentence was going. It felt like I had "handed off" saying the words out loud to a "subroutine", so my attention could be on what I was reading. Then that "readahead" extended to a whole sentence. And at that point it was like I was so far ahead of what I was saying that I had time to think about it a bit. And then at some point it was like the "reading the words" part got handed off to a "subroutine" too, so my attention could mostly stay on whatever I was thinking

This is something that has been studied and is apparently more common when reading out loud. I have this as well. I can read to my kids and at the same time plan the upcoming day. Pretty neat!

Sometimes I read a book out loud and think about something completely different.

I wonder if reading aloud might be like walking. I can be walking and speaking to a person at the same time.

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