The easiest way to do this is to:
Go to CONTACTS TAB > Client Name > "Profile" tab.
Scroll down to "Email Settings" and enter the email address of the person receiving the invoice in the INVOICE EMAIL ADDRESS field (and do NOT enter the client's email address in this field). That way, when you create and send the invoice, the person paying for the invoice will receive it in their inbox.
When you create the invoice, it will actually be made out to the client's name. So, I would recommend that you "Add a Note" in the body of the invoice giving a 1-2 sentence explanation to the person paying the invoice.
If you didn't want to do that option and the invoice had to be made out to the person paying the invoice, then you could add the person paying the invoice as a contact/client in your contact list and use the "Add Miscellaneous" button to create a custom invoice that is sent to the person paying.