Here is my scenario:
Apache is running perfectly with SSL enabled and certs installed on port 443.
I would like to install nginx as well on the same machine. Both as a functional reverse proxy and a server on port 81 or 8443 (using the same cert used by apache)
I found plenty of solutions with nginx as primary server and apache as secondary. I don't want to mess with my existing apache configuration, so nginx will be secondary for me.