I’ve blogged about possessive formation several times here, and I’m going to do it again. This time, I want to focus on those situations where a thing belongs to more than one person, either separately or jointly. This might be a physical thing, like a house or a car, or an abstract thing, like death or success.
Let’s say that in your novel, two people are killed in a car wreck. Perhaps there is a sentence like “Blake’s and Rhonda’s deaths could have been avoided.” Why is that genitive marker on each name? They didn’t have joint possession of one life, so they can’t possess a joint death, either. Two lives, two deaths. Each name gets the marker of the apostrophe and the S. Even in the case of conjoined twins, there are separate lives and separate deaths. Close together, yes, but separate.
[DISCLAIMER: I am aware, particularly in the medical writing/editing field, that there are different ideas about the phrasing of such things when it comes to “our hearts” or “our health.” That’s for the medical writers and editors to be concerned with. Me? I edit fiction. That’s my focus, now and always.]
Now, what about two people who jointly own a single item? “We were invited to Ben and Jerry’s house for dinner.” (Not just dessert, DINNER!) They own the house together, the same way they used to own their company.* The marker goes on the second name. If there were more people in the list, the rule would be the same; the marker goes on the last name in the list, if everyone owns the item together. “Bob, Carol, Ted, and Alice’s bed hadn’t been properly made in weeks.”
It’s not difficult if you take a moment to consider how the item or concept is shared or owned. If the people in question share it, possess it jointly like the house or the bed in the previous paragraph, only the final name in the list gets the genitive marker (apostrophe S). If they each possess (or are connected/related to) a thing singly, like the deaths in the first paragraph, each name gets the marker.
*You’re not going to catch me. They sold (some say sold out) to Unilever back in 2000.