A customer needed a VPN between Denver and Las Vegas. and Openvpn seemed the best way.
I tried using SME to do a site to site, but its a lot of work since Openvpn isnt included in the Server distro.
A routed tunnel seemed best after testing, and ideally, you want the routes automatically pushed to the users.
If you want two way communication, you need two tunnels with each server being both a client and server.
I'd recommend Endian Firewall if you want it working quickly, or for inspiration - I'd love to see something like this added to SME...