Are you running a web site on the non-SharePoint machine? If not, just point port 80 to the SharePoint server.
If you are running a web site on the other server, use another port and a designation for the SharePoint server. For example, point port 8080 to the SharePoint server, and use sp as the server designation in your DNS settings. A user would then type in sp.mydomain.com:8080 to reach the SharePoint server from the outside.
You would want to make this a secure connection as well.