[SNYPE00]

You arent running Connection Sharing thru windows are you?
If you have any number of computers on a lan, you shouldnt have any problem playing on a server thats outside your network (not really much different than bothe your lan machines going to the same webpage). I do it at home, at work all teh time. No ports to forward unless YOU are running a server.
Can you surf the web from both machines? Sounds like your internal network is not configured correctly. You need to either enable NAT or give all your computers static IPs.
If you dont know how to do this we can help.