"Can humans multi-task?" The simple answer is, "No." The more complex answer is this:
Humans can *pay attention* to only one thing at a time. Folks who claim to "multi-task" are usually just rapidly switching their focus of attention among a group of things.
Now, what about walking and chewing gum? Or being able to carry on a conversation while driving a car? Well, humans habituate actions, such that they don't require attention. In fact, humans can't NOT habituate. Among other things, this leads to mis-use of tools (be they physical or "virtual") when interfaces aren't standardized.