You can add multiple people to a contact by creating a company. This will allow you to add as many individuals as required to your contact. 

1) Create a new contact, and select "Add a company".

2) Fill out the details for your primary contact. 

3) Click on "Add another contact to this company" to add additional contacts. 

1) Add your contacts from a company onto your document templates.

2) Changing your invoices to be addressed to a secondary contact.