Once the check is issued, it is no longer an IRS matter. Checks are issued by the Financial Management Service, of the Fiscal Service, of the Department of the Treasury. The governing regulation is "Indorsement and Payment of Checks Drawn on the United States Treasury," 31 CFR Part 240.
Interestingly enough, FMS says nothing about jointly payable checks in this regulation. However, it does have a rule on checks payable to deceased persons. It's in §240.15.
- If an executor or administrator has been appointed under applicable state law, the executor or administrator may endorse the check, indicating the capacity in which the executor or administrator is acting, and including his/her signature. The bank should obtain and retain documentation of the death of the payee and the appointment of the executor or administrator.
- If no executor or administrator has been appointed, the check should be returned to the IRS for a determination as to "whether, under applicable law, payment is due and to whom it may be made." (240.15(b))
I've included the parts of the regulation that would address a tax refund check. Different rules would apply for recurring payments, such as social security or annuity payments.