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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