Nvidia Dual Displays not being detected

Open nvidia-settings and configure it there.

The proprietary driver does not integrate very well with Ubuntu's system settings.

To configure your two monitors manually,

stop your display manager:

sudo stop gdm

if the process is not found, try ps -ax | grep dm to look for your manager.

press:

Alt+Ctrl+F1

and log in. After that create xorg.conf file:

sudo Xorg -configure

The contents of the file should be written on the screen and the path to the created file should be written also (path depends on how You are logged, as super user or as Your user and just using sudo command)

Edit created xorg.conf.new (it is created instead of xorg.conf, but it is the same):

sudo gedit xorg.conf.new

You should find sections responsible for screen card and monitor. In my case these are:

Section "Monitor"
    #DisplaySize      380   300
    Identifier   "Monitor0"
    VendorName   "AOC"
    ModelName    "919"
    HorizSync    30.0 - 80.0
    VertRefresh  55.0 - 75.0
    Option      "DPMS"
EndSection

Section "Device"
    Identifier  "Card0"
    Driver      "radeon"
    VendorName  "ATI Technologies Inc"
    BoardName   "Radeon 2100"
    BusID       "PCI:1:5:0"
EndSection

Section "Screen"
    Identifier "Screen0"
    Device     "Card0"
    Monitor    "Monitor0"
    SubSection "Display"
        Viewport   0 0
        Depth     1
    EndSubSection
    SubSection "Display"
        Viewport   0 0
        Depth     4
    EndSubSection
    SubSection "Display"
        Viewport   0 0
        Depth     8
    EndSubSection
    SubSection "Display"
        Viewport   0 0
        Depth     15
    EndSubSection
    SubSection "Display"
        Viewport   0 0
        Depth     16
    EndSubSection
    SubSection "Display"
        Viewport   0 0
        Depth     24
    EndSubSection
EndSection

Copy Your monitor and screen sections and paste it somewhere in that file, and name (change Identifier) them Monitor1 and Screen1.

In Screen section corresponding to Screen1 change used monitor to Monitor1

Monitor "Monitor1"

reboot and then try to go to system->preferences->screen and detect monitors.