No idea. I just saw other people having the same problem via a google search. Two displays, nvidia options have single, clone, h span and v span and no dualview. They said they clicked "Force TV Detection" and the option appeared, regardless of the display connected. You just don't want to try it because it'll probably make you reboot.
Edit: You can also just try to enable the display through the windows display properties. I have read that some people were able to use that function. Another person was able to set it to span in the nvidia settings and then rebooted, which then displayed the option for dualview. Other than that, I don't know. Perhaps nvidia has a better answer.