Answer:
It depends. Here's a short summary:
You must send the primary applicant an AAN. [1002.9(f)]
You must tell each person that has a "bad" credit report that you used their credit report adversely. [ยง 615 of the FCRA]
If the joint applicant's credit report wasn't used or is fine, then 1 notice to the primary is sufficient. If the joint applicant's credit report is bad, then you're going to have to send an AAN to each applicant.