Duumviracy

Duumviracy is rule by two people, as with the consuls of the early Roman Republic and, perhaps, among the Black Numenoreans of J.R.R. Tolkien's Middle-earth legendarium.

Each member of a duumvirate is referred to as a 'duumvir.' Duumviracy is a very rare form of government, given individual human ambition and the difficulty of amicably or permanently dividing power between two people.