Human Multitasking to me, only works if the tasks are not complex and don't need complex analysis but mainly execution.
I can fold my laundry and talk basically. I can write a simple HTML form and switch between an email and talking to a colleague. These are things that I trained a lot there is no learning involved just acting on a subconscious level.
Any multitasking within a creative process that is not already broken down to smaller pieces that have well defined boundaries seams counterproductive to me.
that's my experience.