An inetersting question...???

The central nervous system is involved in both tasks. Here nail biting and nose picking are usually done sub-consciously, whereas the task at hand takes up more of our attention.

In case of 2 or more tasks which require more or less equal concentration, we switch our attention from task to task extremely quickly. Humans don't do lots of things simultaneously. Instead, we shift our focus from one thing to the next with astonishing speed. We are not paying attention to one or two things simultaneously, but switching between them very rapidly.