Well, there is no clear-cut answer to this question as, to my mind, it depends on personal preference and learning style as well as on the person's ability to shift from one task to another. Studying multiple subjects gives students breadth, especially if they can see connections and linkages between subjects and take full advantage of similarities and differences between them. What is more, it keeps students alive and alert on the task, which in its turn keeps them from getting bored. Though, I can also see advantages in learning one subject at a time. Sticking to this type of learning increases student's concentration and enthusiasm, allows the fundamentals to sink in, and lets a student feel better organised and more aware of the progress. So, I guess it depends on a person and their preferences.