There is no human ability to mentally multi-task.
For someone who is not a pilot, an easier example may be driving a car. Can't you drive it and talk at the same time? Looks like multi-tasking to me.
However, your point is in fact very interesting. Apparently, human thought processes can be roughly divided in two categories, those that require logic and conscious effort and those that are performed instinctively, without actual "thinking".
And the brain is capable of executing exactly one "logical" thread at a time, but many "instinctive" processes at once.
From that point we can return to the difference between "A" and "B" aircraft philosophy. The "B" aircraft provides tactile feedback (moving throttle, forces on the yoke, stick shaker, rotating trim wheel... etc.) which can be processed by instinctive threads of the human mind. The "A" only shows visual references, in the form of symbols rather than images, that require conscious effort to be processed. And there is only one thread capable of doing that, resulting in overload and confusion....