I have a gay, married cousin. How does one address a formal invitation? Mrs. & Mrs Campbell? Or, simply use their first names Jane & Mary Campbell, for example
Considering they’re not going to see anyone else’s invite and whether or not everyone else’s invite has “Mr. & Mrs.” or “John & Martha” I think it’s perfectly fine to put their first names unless you’re sure they go by “Mrs. & Mrs.”
And if one didn’t take the name of the other, Mrs. X and Mrs. Y
When in doubt, identify them as individuals. It’s always a safe bet, IMHO.