The problem is than humans have single locus of attention an can only do several automatic tasks plus *one* conscious task simultaneously.

Quoting Jeff Raskin:

We humans apparently simulate the simultaneous accomplishment of tasks that require conscious control by alternating our attention between tasks, attending now to one, then to the others (Card, Moran, and Newell 1983, p. 42). You achieve true simultaneity when all but at most one of your tasks become automatic. For example, you can, at the same time, eat a snack without choking, walk without tripping, and think through a mathematics problem to a satisfactory conclusion. (You may also be working on another math problem unconsciously, but by the definition of the cognitive unconscious, you wouldn't notice that you were. I am claiming only that you cannot simultaneously work consciously on two different math problems.) For most people, all of the tasks, except for finding the solution to the mathematics problem, are so well learned that they undertake these tasks on autopilot. However, if you were practicing these simultaneous activities and suddenly discovered a nasty-tasting morsel in the snack, you would become conscious only of what you were eating. You would no longer be conscious of the mathematics problem.