This can be done by:
- Navigate to Families> Manage Families
- Search for and select the family
- Click on the blue tab at left
- Select 'More Options'
- Select 'Update Parent & Student Info'
- Enter a 'Secondary Email'
- Select 'Mirror'
- Scroll the the bottom and click 'Save Changes'
Additional information can be found here: What is the difference between Mirror and Supplemental for email addresses?