The Problem with Co-Presidency

"James Wilson of Pennsylvania made the argument for a single officeholder with typical depth and precision: 'To control the executive, you must unite it. One man will be more responsible than three. Three will contend among themselves till one becomes the master of his colleagues. In the triumvirates of Rome, first Caesar, then Augustus, are witnesses of this truth. The kings of Sparta and the consuls of Rome prove also the factious consequences of dividing the executive magistracy.'
"Wilson and his allies carried the day; and their argument is as good now as when they embedded it in the Constitution."

Garry Wills explains why "Two Presidents Are Worse Than One" in The New York Times.