To add a second email address to a customer account, click on "Edit" for their Account Information, and edit the CC Email field. This field can have multiple email addresses, separated by a comma.
You can also add a CC Email address to each of your Email Templates that get sent to customers. From the Configuration > Email Templates, hover over the menu dots by your choice of template to click on "Settings." You will be able to add a CC address for each template there. If you would like to convert the email template CC into a BCC, this can be toggled under Configuration > Site Settings, in the Advanced Settings section.