This was addressed to a family regarding what they should talk about at dinner that night. Should "you" be "your"? What is the rule?
Ted's and your dinner conversation tonight was so boring.
According to CMOS: Joint possession is shown by a single apostrophe plus -s only when two nouns are used. If a noun and a pronoun are used to express joint possession, both the noun and the pronoun must show possession. For example, Hilda and Eddie’s vacation becomes (when Eddie has already been mentioned) Hilda’s and his vacation or (if Eddie is speaking in first person) Hilda’s and my vacation.
Although using possessives for both parties is technically correct (as in "Ted's and your conversation"), this usage is not common and has a formal connotation. I would advise instead to reword the sentence to something like
Regarding your conversation with Ted this evening...
This is grammatically correct while maintaining a less formal feel.