The most likely scenario under which that could happen is if BO were to get re-elected this year and HRC were to run and be elected in '16. Our country cannot survive that. :(
Another possibliity, I suppose, would be for BO to pull out of the race now and HRC to run and be elected instead.
Or, I suppose, if HRC runs now as BO's VP, and he doesn't finish a second term.
I have to admit that the second and third scenarios are not quite as terrifying as the first.