Most people are familiar with using an apostrophe-S to show possession. Consider this sentence.
“Bob’s dog is old.”
In this sentence, Bob owns or possesses the dog. Simple.
Where this gets tricky, however, is when you have multiple owners of multiple things. Let’s say that Bob and Mary each have one cat and that those cats are fighting. Where would you put the apostrophe-S?
“Bob’s and Mary’s cats are fighting.”
Since Bob and Mary each have an apostrophe, we know that they each own a cat individually. Bob has a cat, and Mary has a different cat. We’re using one apostrophe-S for each individual owner.
Now let’s say that they own the cats together and that the cats are still fighting. How would you punctuate that?
“Bob and Mary’s cats are fighting.”
We have used only ONE apostrophe-S to show that Bob and Mary are a group and that they own the cats together.
Here’s the point: Use only one apostrophe-S for each owner, whether an individual or a group.
Just for fun: What would this mean?
“Bob and Mary’s cat is fighting again.”
Here, Bob and Mary own a cat, and that cat is fighting something. Now, how about this?
“Bob and Mary’s cat are fighting again.”
There is only one cat. At first, it seems that the cat is owned by Bob and Mary together, but then the sentence has the verb “are.” The conclusion is that Bob is fighting with Mary’s cat! (Poor Bob)