Think about the taste of chocolate (that glorious silky rush of sweet earthy flavor) at the exact same time as you add 47 and 56. Really try. At the same time. If it makes your brain fuzzy in the way your mouth feels after you’ve had an unripe banana, you’re in good company: it’s impossible. You can switch back and forth really quickly, but you can’t actually think about both things at the same time.
Aza Raskin is showing you with two easy tests that no human being can do „real“ multitasking. The better way to get your tasks done faster is to do them in sequences.
But not only the speed is important. I think sequencing is better for the overall feeling, too. If you can focus on one thing for a period of time, you´ll have less errors in the end. Less errors mean more happiness. Happiness equals motivation. Motivation leads to better work. Got my point?