Setting xorg.conf to mirror mode rather than twinview mode

To deal with my screen resolution problems (Previous post: Setting screen resolution in fvwm running ubuntu 12.04), I found a new nvidia video driver, i.e., an nvidia GF119. While I now get fabulous video resolution on my external monitor, it comes up in what I think is called Twinview mode, i.e., you can drag the mouse from the laptop monitor to the external monitor. Moreover, I can only access the middle two of my fvwm screens (i…e, have 3x2 screen configurations) . Presumably I can fix this problem in xorg.conf, but I have been unable to do so. I tried the obvious thing

Section “ServerFlags”
Option “TwinView” “0”

But that did nothing. I’ve looked at a post which seems to be exactly on this topic

but the response is completely unintelligible

Could somebody advise please? Thanks very much, Leo