internal resources accessible on public IP

i know there's a more efficient way to word this but here goes...

i have a server running RRAS, DNS, DHCP and IIS 6.0 (yes, i am aware there's a newer version). i have 1 public IP which the LAN uses for internet access. i have 2 internal machines that i would like to allow outside access to for HTTP. one uses port 80, the other is on another port. obviously if i also have a website on the main server using port 80, then it would not be available to forward. in RRAS, right-clicking the public-IP's adapter and going to Services and Ports is where you set up forwarding rules, and there is an 'incoming' and 'outgoing' port for each entry. i would like to figure out how to either allow direct access to the specified LAN machines, or perhaps there is another way, where one could use 'public-IP/directory' to view the internal resource 'via' the public machine? i really hope i'm not confusing anyone. thanks!