Diarchy
Diarchy, duarchy or duumvirate is a form of government based upon corule, with two people ruling a state together. It was a style of governance mostly active during the ancient times, examples of this are the Roman Republic or Ancient Sparta. Diarchs often hold their office autocratically and pass on their duties and titles to their children or an otherwise appointed successor. The two Diarchs are not necessarily equal, merely both performing the duties of a Head of State, and both holding titles that recognize them as leaders of the State. A well known modern example is the Principality of Andorra, where two Princes - the President of France and the Bishop of Urgell - rule as Princes and Diarchs.
Diarchies are not exceptionally common micronationally; however, there are several examples of such states. A Diarchy is a favorable system of government for a micronation with two founders who desire equal representation and power over their creation.
Examples
Scientopia is an example of a democratic diarchy.
The Empirical Dyarchy of Ohio is an example of a half-democratic, half-hereditary Dyarchy/Diarchy.
The Salarian People's Republic Is De Jure an Diarchy
Empire of Barbettia is another example of a diarchy. The Empire has its own constitution, with two equal heads of state (kings).
The Republic of Schruįhka is a Diarchy consisting of two self-elected Diarchs, each with equal power; Rigga and Jitt.