In order to be able to have two servers running on port 80 behind a firewall with only one IP address for your connection, you'd have to setup a virtual host on the main web server to point to the other server running apache, example like this:
RewriteRule ^(.*)$ http://192.168.1.10$1 [P]
Where the servername can either be a virtual domain of a main domain your you could specify a totally different domain like www.domain2.com..
but basically whatever is configured on your other internal server.
If you don't have a domain name, no, you can't access two different servers with one IP using the same port, you'd have to switch your router to point to one or the other when you wanted to connect to it.