How to send an email out to secondary Email

Our organization is split into multiple age groups and often has two parent contacts. I would like to send an email to all contacts under one list. How do I choose the secondary contact email to be added to the list. If I can't do it on Constant Contacts, is there another platform I can use for this. If we can't send to multiple contact emails, then this really won't work for our organization.  Thanks in advance for your help.