Yes, Franz, you can add multiple email addresses to a customer in QuickBooks Online so that both recipients receive the invoices.

In QuickBooks Online, you can easily add multiple email addresses to a customer's profile or specify additional recipients when sending a particular invoice. Adding multiple email addresses directly to the customer's profile ensures that invoices are automatically sent to all listed emails every time you email that customer.

For a detailed walkthrough, here's how:

1. Go to My Apps and navigate to Customer Hub.
2. Select Customers.
3. Find and click on the customer's name to open their profile.
4. From the Customer Details tab, choose Edit.
5. In the main Email field, enter both email addresses, separated by a comma and a space.
6. Click Save.

I've included a screenshot for your reference:

Alternatively, if you need to send a specific invoice to an additional recipient only once, you can enter the extra email address directly while creating or sending that invoice. 

Here's how:

1. Open the invoice you want to send.
2. In the To email field on the invoice itself, include a second email address, separated by a comma and a space.
3. You can also enter the second email address in the Cc/Bcc section.
4. Select Send.

Please feel free to update me on your progress or ask any questions as you go along. We're always here to support you.