Here are the general rules to show joint or separate ownership.

To form the possessive showing joint ownership, add an apostrophe and an -s (-'s) to the last noun.

Example:

• Daddy and Mommy's new van.

To form the possessive showing separate ownership, add an apostrophe (') or an apostrophe and an -s (-'s) to each noun.

Examples:

• my friend's and my classmate's houses.
• the teachers' and the pupils' folders.