A compound possessive is the possessive form of a series of two or more nouns joined by conjunctions. In some cases, you should write every noun in the possessive form (e.g., “Kim’s and Harry’s”), in some cases only the last one (e.g., “Kim and Harry’s”). Both nouns are possessive when the different entities possess something separately.

When you have two or more nouns that you're making into a compound possessive, add 's to only the last noun if the nouns are functioning as one unit. Margo and Martha's business was booming. This means that Margo and Martha share ownership of a booming business.