I am almost in agreement with superfly on this one. You need unique CD-Keys for each player on the lan.
I do not own a copy of the GoG version of NWN. So my sugestion here is pure theroy. I have no way to test it.
Your game ships with a generic set of CD keys. You also get another set of CD-Keys whey you request multiplayer keys.
You could try running one of the computers with the multi player keys and the other computer with the generic set. In order to get this to work you may have to block the ports on the computer that try to connect to the master server. This way It will not show if someone else is playing onLine with them. Also if some one is playing on-line with then it will not effect you play on a local network.
Here is the information I have on the Ports the game uses.
If you are trying to connect to a Neverwinter Nights server through a
firewall, NAT, or router, here is some information for you to help you get
connected. First, please read the manual that came with your firewall or
router.
Neverwinter Nights uses UDP, not TCP for its connections.
If you think that your firewall is preventing you from connecting to the
game servers, please make sure that the following ports are open:
Ports 5120 through 5300
If you are wanting to make sure that your NAT is set up to allow the game
to play here are some details:
Outgoing packets:
Source port: 5120-5129
Destination port: 5121-5300
Incoming packets:
Source port: 5121-5300
Destination port: 5120-5129
On the Game Client side, you can select what port your client uses. Go to
your nwnplayer.ini file in your NWN Client Beta install directory. Find the
following section:
[Profile]
Client Port=5120
Change this number if you need to force Neverwinter Nights to connect as a
client on a different port.
Gamespy
-------
If you want to get a server listing via GameSpy, you’ll need additional
ports to be open: 6667, 80, 27900, 28900, 29900, 29901, 13139, 6500
I am not sure wich port you will want to block to block the master server. You may just want to lock the firewall up tight untill yoou get past the master server verification.
Also keep in mind that you will need to set one of the computer to use a different client port.
Hope it works for you. unfortiantly, I have no way to test it.