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I was always taught that if you want to make sure that the possessive pronouns are correct when referring to more than one person, you should try to say the sentence alone and use that pronoun. My boyfriend's favorite song is... Sounds good right? I favorite song...not so good. So I would suggest stating the sentence like this. My Boyfriend's and my favorite song is... Still sounds a bit clunky, but I believe this is grammatically correct. What do you think?