• The kids' toys
• My parents' house
• The teachers' lounge
If two people own one thing, add the apostrophe and s to the second person only.
• John and Mary's new house
• David and Sue's wedding
• Tom and Doug's car
If two people own separate things, add the apostrophe and s for each person.
• Susan's and Beth's books
• Jean's and Dan's pants
• Ben's and Jim's offices