In the Society for Creative Anachronism, rulers of a kingdom are chosen by tournament. Each entrant in the tournament must designate a person as "inspiration", who will serve as consort if that entrant wins the tourney; fighter and inspiration must be of opposite sex. Sovereign principalities must have one prince and one princess. Baronies may have one baron, one baroness, or a baron and a baroness, but may not have two ruling nobles of the same sex.
Debate about this policy within the Society generally follows the lines of the gay marriage debate. Supporters of the status quo often add that there's no historical basis for two men or two women in a romantic relationship serving as rulers of a medieval kingdom; opponents of the status quo concede this point, but argue that the policy should be changed anyway, for reasons of basic fairness.
I believe that the gay marriage analogy is flawed, for a simple reason: there is no requirement in the SCA, tacit or explicit, that ruling nobles have any sort of romantic or sexual relationship. If Duke Hypothetical, having divorced Countess Mundania, chose to fight for his grown daughter Lady Hypothetica, the populace's response would be, "Why, isn't that sweet!" not "Incest is disgusting! I can't subject my children to this filth!" I've heard young men advised to choose a long-time SCA member, someone old enough to be their mother, as inspiration (to provide grounding experience to a reign). And I've known multiple pairs of ruling nobles and fighters/inspirations who were not involved romantically and not ever planning to start.
Thus, the appropriate historical question is not
"Did two men or two women in a romantic relationship serve as rulers of a medieval kingdom?" The appropriate question is, "Did two men or two women ever share ceremonial rulership of a medieval kingdom?" (I specify ceremonial rulership since, in the Middle Ages as in the Society, the precise balance of real power varied based on time, place, and circumstance.) Did
two men or two women ever share ceremonial rulership? Absolutely! Those of us raised on English fairy tales often assume that a king's oldest surviving son must inherit the kingdom; but in much of medieval Europe, succession by primogeniture was neither obvious nor inevitable. Kings, counts, dukes, princes, and emperors often chose to crown a successor while they were still alive to pick one. The chosen co-ruler was often a son, brother, or spouse of the first ruler, but not always: sometimes an ambitious person finagled a co-rulership, and sometimes co-rulership solved a territorial dispute.
A random selection of co-rulers in medieval Europe follows.
- In June 1170, Henry, the second (and then oldest surviving) son of Henry II and Eleanor of Aquitaine, was crowned king by the Archbishop of York. He is generally known as Henry the Young King; since he died before his father, he never reigned alone.
- French kings who ruled with their fathers include Hugh, son of Robert II (eleventh century), and Philip, son of Louis XI (twelfth century)
- Several of the English kings of Kent ruled jointly with fathers, sons, brothers, or nephews.
- Harold Hardrada forced his nephew Magnus the Good to share the kingdom of Norway with him.
- In the Italian principality of Capua-Benvenuto, brothers as well as fathers and sons shared the title of prince (The New Cambridge Medieval History notes, "On one occasion, very briefly in 939-40, this meant that there were no less than four persons using the princely title: Landulf I, two of his sons and his younger brother, Atenulf II.")
- The German region of Oels-Cossil had brothers ruling as co-princes in the fifteenth century.
- A thirteenth-century treaty made Andorra a principality governed by two unrelated co-princes, the Count of Foix and the Bishop of Urgell.
- In the tenth-century, the duchies of Naples and Amalfi had father and son co-dukes.
- Father/son pairs served as co-counts of Flanders (twelfth century), Toulouse (tenth century), and Macon (tenth century).
- Pairs of brothers shared countship in medieval Catalonia (see Ch. 8 in The Experience of Power in Medieval Europe)
I have listed western European examples above. In the Roman empire, and later in the Byzantine empire, multiple emperors were quite common: examples include Justin and Justinian, as well as the two empress plus emperor combination of the sisters Zoe and Theodora together with Zoe's husband Constantine IX. (See Byzantine Empresses: Women and Power in Byzantium