Don’t worry. Here’s how compound (or joint) possessives work:
But if two or more people share ownership of the same thing, and you want to use a possessive pronoun(e.g., my, your, his, her, our, their) instead of a person’s name, it should look like this:
TRANSLATION: When someone looked inside a toilet, he or she found two things:
- My friend (Patrick Star)
Therefore, use the possessive [’s] after the person’s name.
And so, Anon, ”My friend’s and my favorite book” is correct.