Multiple PPTP connections from a single source address does not work when connecting to a PIX or a VPN3000, it does work when connecting to a router though.
There's no specific reason why it doesn't work other than the fact it didn't work at one point and has never been fixed. Call it a bug if you like. There is some specific coding that needs to be done so the PIX/VPN3xxx can detemine which end client the traffic should go to, considering they all seem to be from the same source address, and basically this has never been done, probably due to other features, time restrictions, lack of business need, lack of customer requests, etc.
Sorry I can't give you a better answer.