Attention on multiple tasks
You can definitely pay attention while doing multiple tasks. It's just that the ability to really pay attention all depends on the person himself or herself. In fact, you might be doing just that without your knowing.
For example, you might be watching television and doing your homework or reading a book at the same time. Another example is you might be cooking 2 different things at the same time.
Although you can pay attention while doing multiple tasks, you will notice that your attention is not as good as when you're focused on only one thing. That is of course natural. It all depends on how well your brain processess information. Although your attention is not as good as when focused on only one thing, you have to admit that you can get many work done and in this way, I feel that your memory load is also reduced. What it means is that when you're focusing on many things, you tend to try and remember what you're doing then and not remember it for the long term. Thus, there's not much pressure on your brain or mind to remember everything to the last detail. Rather you just have to remember what you're doing at the moment.
Researchers have found that learning is effective when you split attention between different sources of information. In fact, there are different types of attention. Investigation is still going on to determine the source and effects of signals that generate attention and also the relationship between attention and working memory. Investigation is also ongoing on traumatic brain injuries and their effects on attention.
You can also find out more at http://en.wikipedia.org/wiki/Attention