this is how i have enabled Twinview long time ago.
and it works without any problems since then.
in your xorg.conf or 15-nvidia.conf (or whatever you named it in /etc/X11/xorg.conf.d) in the device section you have to put lines like these:
Option "TwinView" "1"
Option "TwinViewXineramaInfoOrder" "DFP-0, DFP-1"
Option "metamodes" "DFP-0: 1680x1050 +0+0, DFP-1:nvidia-auto-select+1680+0"
Option "TwinViewOrientation" "DFP-0 LeftOf DFP-1"
in my setup i have the external monitor right of my laptop display.
the external monitor is a DVI connected LCD, therefor it is referenced as "DFP-1".
my internal display has a resolution of 1680x1050.
you only have to change the values to your needs.
this should work.