I have set up a server running Windows Server 2008 which two remote users can dial into using a VPN connection. It works fine unless they are both in the same location, in which case only one of them can connect.

I understand this is because the PPTP protocol cannot cope with two VPN connections from the same IP address. Is this correct? If so, what can I do about it?

Please note that the remote location in question is a serviced office, so we're not in a position to change or play around with their router.

Thanks!