No worries, man. We’ve all been there
You can, but the first one is (most likely) using the 443 port, so the 2nd one will use a different port, with the side affect of needing to state the port number in the URL.
So let’s say you’ll forward 4433 to the 2nd NAS on your router. You’ll need to enter
mynewsite.com:4433
to browse to it.
The other option is to play with reverse proxy and change a few things on the first site and make them use a subdomain. Something like:
firstsite.mydomain.com
mynewsite.mydomain.com
Both will use the reverse proxy with 443, but traffic will be handled by one (the one with the reverse proxy).
If it’s a personal site, the first option is easier. If it’s commercial, having ports at the end will not look professional and might sound suspicious to a lot of people.