The Collaborative International Dictionary of English v.0.48:
Diarchy \Di"arch*y\, n. [Gr. di- = di`s- twice + ? to rule.] A form of government in which the supreme power is vested in two persons. DiarialWordNet (r) 3.0 (2006):
diarchy n 1: a form of government having two joint rulers [syn: diarchy, dyarchy]