Some time ago, in the late ‘70s or early ’80s, I was transcribing a longhand draft by typewriter. There were no word processors then. Typically, one reads the original and types the copy simultaneously. Diane came in, and we had a brief conversation, which did not interrupt my transcription. Which, in turn, did not interfere with our conversation. I was engaged in four different verbal activities at the same time — reading and writing, and listening and speaking. When Diane left, I continued my transcription, and had the simultaneous thought that this was pretty neat.
I came across an article some years later, about the verbal areas of the brain, illustrated with brain scans, which showed that reading, writing, speaking, and listening were, indeed, in separate if closely connected areas. Each area had its own verbal function. So my observation of some years previously was validated.
It can be more complicated than the article showed. I have, on occasion, listened to two conversations at once, though neither with as much comprehension as if I were listening to just one. I have been able to respond to two (rarely three) different conversations in sequence without pausing, first this person, then that. But I am not (some people would think fortunately) actually able to say two different things at the same time.
I have not tried reading two books simultaneously. I can easily switch between three or four sequentially, without losing my place in any of them. I have always been a slow reader anyway. I read in real time, hearing the dialogue, seeing the scene, experiencing the atmosphere, and so on. Nor have I tried writing different things with each hand (though I can, with some illegibility, write with my off hand). But I do write and think at the same time, transcribing my thoughts while I have them. Everybody does that, I think.