While it is true that we cannot truly concentrate on more than one task at once, there are still things that we can do that are “related to multitasking” that can make us more productive.
While we cannot concentrate on more than one thing at a time, it is also true that we have a very hard time doing even one thing in a sustained way for a long period of time. We inevitably get distracted by things like stray thoughts running through our heads. This can reduce our productivity even when we are trying to be as productive as possible on one task at a time.
One thing that we can do is to be receptive to these stray thoughts and to have a mechanism for using them productively. For example, let us imagine that I am lesson planning for a government class I am teaching. I cannot, at the same time, lesson plan for my history class. That would be multi-tasking and I cannot really do both. But what happens when something in my government class reminds me or makes me think of something that would make sense for my history class? I need to be ready to make use of that thought. I need to do something like having a note pad ready on which to jot down any such thoughts that come into my head. That way, when I can get that thought down on paper where I can use it when I plan my history class.
This, of course, only works for creative tasks. For tasks that are more oriented around making things, all we can really do is to engineer and organize our tasks so that we can do something on Task B while waiting for something to happen on Task A. For example, when I worked at McDonald’s, we would put the buns in to toast and then turn to putting the meat patties on the grill while waiting for the buns to toast. All the tasks for making burgers were engineered and organized so that we could do multiple tasks since each task did not require our full attention at all times.
Despite frequent misconceptions, due to inaccurate use of the term, human brains cannot actually multitask. What appears to be multitasking, e.g. someone (illegally) texting and driving at the same time is actually fast task switching. The reason this reduces productivity is that (1) it takes several seconds to switch between tasks, and that leads to lost time (2) the lack of sustained attention reduces performance. This leads, in the work environment to the problem of throughput vs. turnaround. If, for example, you are writing a report and answering texts and emails, you will be able to get better turnaround on the texts/emails by frequent task switching, but the report will take more time to complete and probably be lower quality. Many businesses address this by suggesting that employees organize their work into blocks, by, e.g. working on the report for an hour, doing something healthy like taking a short walk around the building (which can improve creativity), and then answering emails.