Two demanding things at once is a story we tell

What feels like parallel work is fast alternation with a fee attached.

The comfortable belief is that some people can genuinely run two demanding things at the same time. What actually happens is rapid alternation: a slice of one, a slice of the other, back again, each hop carrying its own small cost. It feels like parallel because the hops are quick. The output tells a different story, usually in the form of small errors in both pieces of work and a strange inability to describe either afterwards.

This does not apply to everything. Plenty of activities pair perfectly well because only one of them makes real demands: walking and thinking, tidying and listening, driving a familiar route and holding a conversation. The trouble comes when both things need the thing there is only one of. Then you are not doing two jobs, you are doing one job badly twice while feeling unusually busy.

The reason the belief persists is that the feeling of alternation is genuinely pleasant. There is a lift in it, a sense of being on top of everything, and that feeling is not correlated with the quality of the result. So the experience says one thing and the work says another, and most people go on trusting the experience because it is the one they can feel while it is happening.