Hi,
It really doesn't matter if it's in your closet on a server or at home, if both of your website have a VHOST record in your, and I will assume here Apache, maybe IIS, with a VirtualHost Record you don't have to forward at all to multiple ports, lets assume you have a port :3999 and that was your Apache port.
You then have DynDNS for 2 domains lets just call them "steve.dyndns.org" and "john.dyndns.org" and they goto your IP, and your router does the forwarding work 80 to 3999, all that you need to ensure is your Apache has VirtualHost records for both john and steve's domains, then it will resolve the right page, just like any other server, and all through the same port.
1. Setup Virtualhost records for both subdomains you want to use or domains
2. Point them to their site locations
Now when they resolve to your IP and port, they will find their home.