This occurs when printing report cards. 
Both parents have the checkbox for Receives report card marked on their record. 
Report cards are set to use address information from the Student record and not the Relationship. 

The parent will need to be listed as a Relationship in the students record. 
Also, in the Student record, under the Relationship tab, both parents will need to have the checkbox for Receives Report Cards marked. 

1. In Mail, open the report card parameters.
2. Select the Address tab (#6).
3. Select the drop down box by Use address from: and choose Relationship.
4. Leave the checkbox for Combine for each spouse pair unmarked.
5. Rerun the report card. 

This will allow you to print a separate report card for both parents.