Re: Roman Emperors, I’m not sure that’s fair. Nearly supreme executive power was also vested in the Republican consulate, and that system worked fairly well for several centuries, with the added distinction of generally peaceful transitions of power. It only broke down in the last half century or so of the Republic. And prior to the Tetrarchy there was a long history of having two rulers, one senior and one junior, until the legionary influence on the succession became more pernicious.