Often one can get confused linguistically in terms of royal watching in a language other than English.
In Spanish, whenever the King and Queen attend something together, they are referred to as the plural of King in the Court Calendar of Casa Real. (Los Reys).
The logical step for many Spanish speakers when they do the translation, or the automatic translation, therefore, seems to be to refer to them in the plural in English also. This can lead to confusion, when someone uninitiated to the practise, looks at pictures, where we’re told that the kings are, but we only see one King.
But you also have it for the younger generation. Whenever Letizia and Felipe do acts together, they act as the plural of Felipe’s title, instead of as two separate linguistic titled entities, as the Prince of Wales and the Duchess of Cornwall does. (Or the Prince and Princess of Wales…)
A bit sexist, perhaps, but it seems to follow the norm in the language.
Scandinavian languages also have the common term for couples. You have kongeparet, kungaparet (both meaning the King Couple, in Norwegian and Swedish respectively) and regentparret (the regent couple) in Denmark. One could wonder whether kongeparret (the king couple) will return to the Danish language when Frederik and Mary ascend to the throne, or if they will keep the more neutral version of today.
The only way English seems to incorporate the plural, is when it has something that can be officially made into the plural – to put it like that. It is Their Majesties and Their Royal Highnesses… or when non-native speakers decide that the traditions they’re following from their native language also belong in English, such as Kings. 😉