Using Mac OS X Server to make a site-to-site link was theoretically possible with Mac OS X 10.6 Server. It was done via a command-line option called s2svpnadmin
See http://webcache.googleusercontent.com/search?q=cache:5eH5QIThXLYJ:docs.info.appl e.com/article.html%3Fpath%3DServerAdmin/10.6/en/ns212fab54.html+mac+os+x+server+ vpn+site+to+site&cd=1&hl=en&ct=clnk&gl=uk&client=safari
Note: Apple seem to have removed a lot of old (even Lion) documents from their site so I am posting a Google cache URL above, however even the Google Cache will loose it soon.
You could in Lion or Mountain Lion Server run the following command to see if the s2svpnadmin utility is still available
man s2svpnadmin
Unfortunately Apple have been dumbing down their server software so that while it is becoming simpler to setup it is also becoming far less capable. 😟
Personally, I would consider using a hardware device for doing site-to-site VPN links. Most business class Firewalls have VPN capabilities to do this. Most consumer level routes/firewalls do not.