Hola Sanchezluis:
Me refiero a esto:
En http://www.pfsense.org/index.php?option=com_content&task=view&id=40&Itemid=43 tienes:
PPTP and GRE Limitation - The state tracking code in pf for the GRE protocol can only track a single session per public IP per external server. This means if you use PPTP VPN connections, only one internal machine can connect simultaneously to a PPTP server on the Internet. A thousand machines can connect simultaneously to a thousand different PPTP servers, but only one simultaneously to a single server. The only available work around is to use multiple public IPs on your firewall, one per client, or to use multiple public IPs on the external PPTP server. This is not a problem with other types of VPN connections.
No se pueden establecer dos vpn pptp simultaneas contra un servidor externo debido a esta restricción de los firewalls basados en packet filter.
También se hace referencia a este problema en este post del foro: http://forum.pfsense.org/index.php/topic,1383.0.html.
Saludos