Before I got to playing with Freeswan in the past, I have managed to get multiple workstations behind a SME 5.1.2 server to connect to a MS PPTP server (as the company with MS end does not and will not support IPSec, at least in the forseeable future). The answer then was to get SME to be the PPTP client, and not the workstations themselves (hence, only one PPTP connection is being made). This was accomplished by installing the Linux PPTP client module into SME and configuring it to talk to the MS PPTP server at the other end. The workstations behind SME connects to internet as normal and has access to the remote network as well.
Kelvin