Coregency
A coregency is the situation where a monarchical position (such as prince, princess, king, queen, emperor or empress), normally held by only a single person, is held by two or more. It is to be distinguished from diarchies or duumvirates such as ancient Sparta and Rome along with a regency where the monarch in most cases infant or incapacitated particularly has a senior above him and exercises no power whatsoever apart from legally holding the position of head of state.
This article is about multiple sovereigns of a single state. For a single sovereign shared by multiple states, see Personal union.