I had the game, but a disc was damaged. I still have the codes. How do I put the code into the game.? I want two games so my kids can play together

Is this the first NWN? If so, I think the CD key is stored in a text file named nwncdkey.ini within the game installation folder. Edit that file for your other key that you have. Afterwards you will probably have to run the Premium Modules Re-Installer in order to make the addon modules work with the key you entered.