Simpler. Create a new xp user account. Make a back up of all the NWN player files. Uninstall the game. Reinstall your game telling the game not to install for all users. Imput your cd key. and put your player files back where they were. This is to ensure you keep your journal intact (what ever notes you took in teh journal while in the game)
Go buy an other copy of the dimond set box. Go into the newly created account for the other player, install the game in a diferent folder, telling it not to install for all users, enter the new cd keys.
Now you have twice the game installed with the two diferent cd keys. Do the first step that was told to you previously as to save his player under the new cd key and voila.
When the other player will want to play he will only have to go onto his account, there wont be the change to one file and back to the other when ever you two want to play.