But don't we all know that not to be true? This is clearly evident with training sports, learning to play an instrument, or even forcing yourself to start using your non-natural hand for writing — and really, anything you are doing for the first time.

While we are adapting our brain to perform a certain set of new actions, we build our capability to do those in parallel: eg. imagine when you start playing tennis and you need to focus on your position, posture, grip, observing the ball, observing the opposing player, looking at your surroundings, and then you make decisions on the spot about how hard to run, in what direction, how do you turn the racquet head, how strong is your grip, what follow-through to use, + the conscious strategy that always lags a bit behind.

In a sense, we can't really describe our "stream of consciousness" well with language, but it's anything but single-threaded. I believe the problem comes from the same root cause as any concurrent programming challenge — these are simply hard problems, even if our brains are good at it and the principles are simple.

At the same time, I wouldn't even go so far to say we are unable to think conscious thoughts in parallel either, it's just that we are trained from early age to sanitize our "output". Did we ever have someone try learning to verbalize thoughts with the sign language, while vocalizing different thoughts through speaking? I am not convinced it's impossible, but we might not have figured out the training for it.