You can configure TARs to send email notifications to the technician every time the client responds to a ticket via email or via the portal, but you can't configure TARs to send email notifications to the client every time the technician responds to a ticket via email or the portal. The closest thing to this is enabling the feature "Send ticket reply to contacts by email" in the Email Settings, but that's aesthetically awful as with each reply snowballs into a long thread and becomes unreadable.