Use the apostrophe + s after the second name if two people possess the same item. Otherwise, use an apostrophe after each name.

Rule #2: Use an apostrophe + -s at the end of the second proper noun when two people jointly own the same item together. When two or more people have joint possession of an object, follow the rules above that places the apostrophe -s at the end of the last noun. The same rules apply to nouns already ending in -s as well.