Multiple PPTP passthrough sessions

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I am running a SBS 4.5 server with ADSL connected through
a Linksys router on a second NIC. There two remote
offices which are also connected to ADSL via a Linksys
router. The remote offices have one computer behind the
router, and four at the other. The clients run win2000
and use VPN to connect to the Server, PPTP protocol.
Routers are all switched to PPTP passthrough on and also
port 1723 forwarded.

All works fine when one client connects, but if a second
one connects then one will time out. I have since learnt
that the Linksys routers will only handle one PPTP
passthrough session at a time! I was then told to buy
the Linksys VPN routers as they can handle up to 70
tunnels, but I was not told that was IPSEC tunnels, and
the same problem was experienced with these routers.

Can you possibly give me some explanation, point my in
the right direction or help me with a solution? I have
been trying to get help for months to no avail..