Simply create new virtual hosts. This is done in the Server App. Apple calls them Web Sites. Simply define a new site, setting the fully qualified host name in the domain name field. Then select the site root in the Store Site Files In field.
The Apache web server will read each http request. The request will contain the host name that the user is asking for. Apache will parse that information and route the viewer to the correct virtual host.
Actually, this has been working for me as long as I did not use SSL.
When using SSL, all requests were pointing at the same domain name. I have tried to use two different Ethernet interfaces with two different IP addresses but it did not work either and always pointed to the domain name with the smaller IP address.
Any help would be welcome.