Multiple PPTP VPNs inisde the LAN

I am securing my wireless WEP connections by installing a
PPTP VPN on them, and firewalling each computer. Note
that I'm not VPN'ing from the WAN.

I have however been unable to run two PPTP VPNs
simultaneously on two computers (one for each direction).
Does the MN500/520 limit the number of PPTP VPN
connections inside the LAN? Am I doing something wrong?

Is there a better/simpler way to VPN the wireless
connections?

Thanks