Ancient Sparta’s two-king system represented a diarchy.
A diarchy is a system in which two rulers share supreme authority. Sparta’s kings came from two hereditary royal houses, the Agiads and the Eurypontids. Each dynasty claimed descent through a separate line, and both kings held office at the same time rather than succeeding one another as a single monarch.
The kings had important religious, military, and judicial responsibilities. In wartime, a king could command the army, although the scope of royal authority was limited by other Spartan institutions. The ephors, the Gerousia, and the citizen assembly all constrained royal power in different ways.
Calling Sparta simply a monarchy misses this institutional balance. It was also not a democracy in the modern sense: political participation was restricted, and many residents lacked citizen rights. The term oligarchy can describe some features of Spartan government, but diarchy specifically identifies the distinctive dual kingship.