Hi carolbkelly,
The information within the header section of the custom invoice template (business name, phone, email, address) is pulled from the Company Settings, in which it is not possible to have multiple customer-facing email addresses.
If you update the email in one template, this will update across all templates and within the Company settings. You leave feedback for this by going to the Gear icon > Feedback within your account, which will go directly to our software developers :)