grammaticality - Preferred way to apostrophise in case of dual or multiple ownership by distinct entities

Consider describing the wedding of X and Y. If I want to avoid the overly-formal and poor-flowing "wedding of", it is more correct to say "X and Y's wedding" or "X's and Y's wedding"?

I acknowledge a very similar question has already been asked: What possessive forms are used for mutual 1st person ownership?

But unfortunately the example given is able to be easily phrased a different way and so respondents have been able to get away with avoiding the direct question.

Let's face it, we come across the need to get across the concept of mutual possession all the time and we don't always want to have to resort to the more tortured "Z of X and Y" as with the French language.

NB: In some cases, people get around this problem by dropping the apostrophes altogether and "adjectivising" the owners, especially if the owners are actually plural entities themselves. Eg. The "Mazda and Mitsubishi combined outputs" instead of the "Mazda's and Mitsubishi's combined outputs". Let's not let this muddy the waters though.

Answer

This site states it very well:

4. A less-often faced decision involves the use of apostrophes where multiple owners are named. Where two or more people own one item together, place an apostrophe before an "s" only after the second-named person. For example:

Incorrect: Bill's and Mary's car was a lemon, leading them to seek rescission of their contract under the state's lemon law.
Correct: Bill and Mary's car was a lemon, leading them to seek rescission of their contract under the state's lemon law.

However, when two or more people own two or more items separately, each individual's name should take the possessive form. For example:

Incorrect: Joanne and Todd's cars were bought from the same dealer; both proved useless, even though Joanne's car was an import and Todd's was a domestic model.
Correct: Joanne's and Todd's cars were bought from the same dealer; both proved useless, even though Joanne's car was an import and Todd's was a domestic model.

("The Legal Writing Teaching Assistant: The Law Student's Guide to Good Writing", by Marc A. Grinker)

So, saying "X's and Y's weddings" (note that it's weddings not wedding) has a different meaning from "X and Y's wedding".

The first one is denoting two separate weddings, and the two subjects named are not getting married to one another, but the second one is the one you are probably trying to say.