co-king
Yass queen
They’re most likely both queens
I reckon he would be a prince.
Usually, if a man inherits the throne, his wife is called Queen, but if a woman inherits the throne, her husband is a Prince.
The reasoning for this is that, back in the day, the king would be of royal blood, and the queen had married into the family. Once this was no longer a necessity, they didn’t want any confusion about which one of the couple was the heir to the throne, so they couldn’t call the man that married the queen the king, due to the assumption of those unfamiliar with the family tree, that he would be the more important one.
That same logic applies to a king marrying a man, you’ll want it to be clear which one is the royal one. Perhaps it would also apply to a queen marrying a woman, making her a princess, for the same reasons.
Prince-Consort.