We think we're multi-tasking, but we're not, really. When we're seeing and hearing different things, we can only take in one channel at a time, so that we're only really see or hearing, not both. So what are 'good multi-taskers' really doing? These folks may either be people who are good at switching back-and-forth, or those with good memories (auditory or visual) so the previous track can be held in mind while switching to the other. In the figure below, attending so what was being heard, shut off what was being seen, and attending to what was being seen, shut off what was being heard.
In kids, we often see this with in the "watching TV-but-didn't-hear-you-call-me" phenomena. Some poor kids are even dubbed "hyperfocusing" (ala 'ADD') just because they are in the strong visual attention mode, but this is probably part of everybody's make up to some extent.
There are some kids and adults who do seem to have more trouble than others performing mixed modal tasks like multi-stepped mathematics (seeing the flow of problems being solved while listening to the teacher's instruction), but that might be due more to problem keeping information in mind (visual memory or auditory memory) than switching problems per se.
It's details like these that aren't obscure neuro-trivia. We see folks who struggle with multi-tasking all the time. If that's the case, then memory should be looked at as well as 'switch'-ability. Most people really aren't taught how to optimize their style of remembering, but it often can be trained just like a muscle, and it can impact almost everything you do.
In the neuro-classroom of tomorrow, we can envision cognitive training becoming as important a part of education as fact and skill mastery.
Yantis Divided Attention paper
Press release: Divided Attention
Windows Media movie about Divided Attention