Most rulers have spouses and many have mistresses (or "misters?") as well. However, often only one of the couple exercises rule and the other is merely to decorate the palace, provide heirs, seal alliances and entertain the king.
A ruling couple, on the other hand, are equal or near equal partners, and may even be Happily Married. Rather then one ruling and one staying in the palace they jointly [...] [tvtropes.org]