No, there can be only one president, so former presidents go back to being “Mr.” (or, I suppose, if there is a female former president, Mrs., Miss or Ms.).
See: http://www.emilypost.com/forms-of-address/titles/118-addressing-a-former-us-president
As a matter of political theory, one could also argue that former presidents do not keep the title the way that former senators, military officers, ambassadors or judges do to assert the republican principle that the presidency is not a kingship, and that the occupant of the highest office in the land reverts back to private citizen after his term is over(in contrast, British prime ministers become peers after they leave No. 10), thereby asserting that this Republic is led by a simple citizen chosen by citizens from amongst themselves for a period of time and not by some special class of persons distinct from the People.
