Communications can be run by different personas. Sometimes there are individual accounts under the same household and users would like the the names for both spouses to be combined in a mailing.
If the joint name is not showing up verify the following: 1) A Joint Persona exists on the account. 2) The communication template is addressed to a Salutation Type, not Account Name. 3) When generating the letters and on the Criteria step select 'Joint' under Personas To Export