Preferred way to apostrophise in case of dual or multiple ownership by distinct entities [duplicate]

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.

"X and Y's wedding" would mean the wedding was between X and Y; "X's and Y's wedding" would refer to two distinct weddings.

Comma sense—the fun-damental guide to punctuation (Richard Lederer and John Shore) contains the following note about the usage of the apostrophe in such cases:

If two or more people possess the same thing, you need only to put the apostrophe after the last one of the two mentioned:

Len and Barry's ice cream business never really took off because all their products tasted like squid.

Trista and Trisha's schemes might have worked had they been twins, instead of other kind.

Larry, Curly, and Moe's inability to go on separate dates was perplexing to some, but not all.