I see. You can manually add the contact details from the customer profile. To do this, click on Sales on the left menu > Customers > click on the customer's name > Edit > enter the other contact details separating them using a comma > hit Save. From here you'll be able to send invoices to multiple emails without having to enter them each time you create an invoice. Give this a try and let me know how you make out. I'll be on standby.