I would expect performance to be disjoint for most tasks. If there was an overlap, the big would probably be less power efficient (as in energy/instruction) than the little so you would prefer to run on the little anyway.
In what way would you use the overlap?
if the scheduler thinks a task would be better off on the other side than where it is now, it could first move it into the "overlap area" on the same side by means of experiment, and if the task behaves as expected there, THEN move it over.