I would suggest looking at the Wikipedia article on Human Multitasking. At least what is there is my understanding of the problems associated with humans trying to do multiple 'active brain' things simultaneously. I'm sure one can train oneself to do a couple things pretty well (like flying and talking on the radio) but other things like reading Plato while listening to an IPOD or watching TV and doing physics problems seem (for most) to suffer the problems noted in the article.