You haven't missed something, @RenaRT. 

Customizing your email templates needs the company file to be in a single user. You must also sign in as admin or have the permission to modify company preferences. With this, there isn't currently a way to switch back and forth between the two templates if you're on a multi-user mode. 

I can see how this feature would be beneficial to your business. Rest assured, I'm taking note of your suggestion for future enhancements. If you're using the 2019 version, I'd suggest visiting this link: What's New. In this article, you can learn more about our latest product features that can benefit you and your business.

You can count me in if you need anything else. Have a good day, @RenaRT.