I got it working thanks to your help!
I found this fantastic post outlining exactly my scenario: multiple Rasperry Pi Apache Ubuntu servers hosting DDNS wordpress websites behind a single home dynamic IP:
It fleshes out very well what you explained to me in this post.
First Apache server serves the webpage it's hosting, and also acts as a reverse proxy server passing on requests for the second webpage on the second Apache server.
As you suggested I have the first server handle all the SSL requests with Let's Encrypt SSL certificates usuing Certbot, and the requests are passed to the http port 80 on the internal network for speed and configuration simplicity.
It's incredible to be able to host multiple websites with no ongoing expenses except power and cost of home internet, and running on inexpensive Rasberry pi servers. For low traffic websites, not having to pay for SSL, Web Hosting and DNS is a game changer.