Whatever Social Security Number (SSN) exists at year-end on the membership is what is used for reporting purposes. CU*BASE only reports under one SSN per membership, and that membership is recorded in the TAXACCTSUM file. CU*BASE will use the SSN that resides at the end of the year.
In the case of a deceased member, where you would like tax reporting to be done under a different SSN for the remainder of the year, it is possible to open a new membership and transfer everything over to the new membership. That way the joint owner, or other person, will receive dividends for the remainder of the year, and the deceased will receive dividends for the portion of the year before their passing.