If you see the desktop in mirror mode on both monitors after you connected the second one, then you just have to do some few steps to get it in extended mode.

 Don't touch the GUI Setting tool (displayconfig-gtk) for monitors, it won't work correctly and may
 destroy your xorg.conf

To see the identifier for your monitors you can use "xrandr -q", see sample output below:

 ~$  xrandr -q
 Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 3200 x 1200
 VGA connected 1280x1024+0+0 (normal left inverted right) 376mm x 301mm
    1280x1024      60.0*+   84.8     75.0     59.9  
    1280x960       84.9     59.9  
    1152x864       74.8  
    1024x768       84.9     75.1     70.1     60.0  
    832x624        74.6  
    800x600        84.9     72.2     75.0     60.3     56.2  
    640x480        84.6     75.0     72.8     66.7     60.0  
    720x400        70.1  
 TMDS-1 connected 1280x1024+0+0 (normal left inverted right) 518mm x 324mm
    1920x1200      60.0 +
    1600x1200      59.9  
    1680x1050      60.0  
    1280x1024      75.0*    59.9  
    1440x900       75.0     59.9  
    1280x800       74.9     59.8  
    1024x768       75.1     60.0  
    800x600        75.0     60.3  
    640x480        75.0     60.0  
    720x400        70.1  

VGA is the identifier for my old 17" TFT that is connected with a VGA cable and TMDS-1 is my new 24" monitor that is connected with a DVI cable.

To find out, which resolution you have to set for the virtual display, you can use following command instead of calculating yourself ;-)

 ~$ xrandr --output TMDS-1 --auto --left-of VGA
 xrandr: screen cannot be larger than 1920x1920 (desired size 3200x1200)

This output tells you the needed size of the virtual desktop that you have to enter in xorg.conf

 sudo pico /etc/X11/xorg.conf

There you go all the way down to the section "screen" and add "Virtual {X} {Y}" below the mode line

 Section "Screen"
         Identifier      "Default Screen"
         Device          "Intel Corporation Integrated Graphics Controller"
         Monitor         "Acer X243W"
         DefaultDepth    24
         SubSection "Display"
                 Modes           "1920x1200" "1680x1680" "1600x1200" "1440x1440"$
                 Virtual 3200 1200
         EndSubSection
 EndSection

Now you have to reload your X-Server

 press <ctrl>+<alt>+<backspace>

Login again and enter the command from beginning in a console window

 xrandr --output TMDS-1 --auto --left-of VGA

You should now have an extended Desktop where the left side of the desktop is on the big 24" monitor and the right side on the old 17" monitor. For xrandr you could also use following arguments for positioning

 --pos <x>x<y>
 --left-of <output>
 --right-of <output>
 --above <output>
 --below <output>

You can add the xrandr command to session startup scripts to have it executed automaticaly or you can try the permanent solution next.

Adding to session startup on gnome

Go to: System > Preferences (Einstellungen) > Session (Sitzungen)

Add a task called "dualhead setup" and use command "xrandr --output TMDS-1 --auto --left-of VGA" or whatever xrandr command you have.

Try to set it permanent with xorg.conf (failed)

 This gave me just the lower resolution (1280x1024) :-/
 I try to fix this but until then you can executed the upper command in a script when x has started

As this is just temporary until next start of X, we will change xorg.conf the way it will setup this by default.

 ...
 Section "Device"
         Identifier      "Intel Corporation Integrated Graphics Controller"
         Driver          "intel"
         BusID           "PCI:0:2:0"
         Option          "monitor-VGA" "WoV"
         Option          "monitor-TMDS-1" "Acer X243W"
 EndSection
 
 Section "Monitor"
         Identifier      "WoV"
         Option          "DPMS"
 EndSection
 
 Section "Monitor"
         Identifier      "Acer X243W"
         Option          "DPMS"
         Option "LeftOf" "WoV"
 EndSection
 
 Section "Screen"
         Identifier      "Default Screen"
         Device          "Intel Corporation Integrated Graphics Controller"
         Monitor         "WoV"
         DefaultDepth    24
         SubSection "Display"
                 Modes           "1920x1200" "1680x1680" "1600x1200" "1440x1440" "1280x1280" "1280x1024" "1024x768" "800x600" "720x400" "640x480"
                 Virtual 3200 1200
         EndSubSection
 EndSection
 ...

And again reload your X-Server

 press <ctrl>+<alt>+<backspace>