You need to use windowed mode, then just set your NWN resolution to something smaller then 3360x1050 but larger then just one monitor (2880x900,2464x768 ) and position the window to the left edge of the left monitor, or right edge of the right monitor to have the player show up fully in one monitor.
I'm not sure if NWN will show each of those resolutions available in Windows, I see them in Linux though. Perhaps they can be forced by editing the nwn.ini file directly, again not sure.
Edit: Yeah what St8 said