VPN troubles

me and my mate are trying to get our two (and more) computers to connect ot each other (to play C&C generals amongst other things) from different routers and internet connections.
He has set up a VPN on his PC, and my PC has managed to connect to this network, but it seems only one connection is allowed at a time.
If i'm connected to the VPN he can not connect, and if hes connected i can not connect.
Does anyone here know how to solve this.
We have tried:
- port forwarding a range of ports on his router
- assigning a port (by specifying a port at the end of the VPN IP address e.g. 12.34.56.78:1234) and connecting to that.
- turning on the option 'negotiate multi-link for single link connections

cheers for any help/thoughts