What do you write when two or more people possess the same thing? Do you use an apostrophe for each of their names or just one apostrophe?
“John and Serena’s car is a bright red.” By using the apostrophe only in Serena’s name, you are signaling that John and Serena both own that car. Use a possessive apostrophe only in the owner’s name closest to the item.
If each one of them owns a separate car, use possessive apostrophes for both owners: “John’s and Serena’s cars have adjacent parking spots in the office garage.”