From what I can tell so far Twinview is not compiled into the nforce2 nv driver thats being distributed. You're in the same boat as I. If you want Twinview to work your going to have to compile the source yourself and enable it at the DRIVER LEVEL.
THEN it's a simple matter of adding the
option Twinview "enable"
settings to your XF86config
Good luck, and let us know if you get it working! there are a ton of nforce2 useres out there that want to see Twinview enabled!