[vpn-help] multiple connections behind a fritzbox to one lancom 1711

[[seniorbook] Dino Dagostino]

Hello folks,
i have the following strange things coming up using the shrew VPN client 
connecting to a Lanocm 1711 router
this is the initial position:
in this case we have two offices. the first has the Lancom 1711 VPN as 
router
the second office is connected to the internet via a Fritzbox 7270

Whe person a in the office initiates a VPN Connection by the shrew 
client all works fine. But when person b starts his VPN session at the 
same time, i does only work at one of the clients.
On the other client not even surfing is possible any more.

I did the config appropriate to the how to on shrew.net with IPSEC

Does anybody have a solution for me? :-)

Thanks in advance

Dino