To use double monitor in extented mode you can use xrandr tool.
To identify your monitor just type xrandr

[onur@onur ~]$ xrandr
Screen 0: minimum 320 x 200, current 1680 x 1050, maximum 8192 x 8192
VGA-0 connected 1280×1024+0+0 (normal left inverted right x axis y axis) 376mm x 301mm
1280×1024      60.0*+   75.0
1280×960       60.0
1152×864       75.0
1024×768       75.1     70.1     60.0
832×624        74.6
800×600        72.2     75.0     60.3     56.2
640×480        72.8     75.0     66.7     60.0
720×400        70.1
HDMI-0 connected 1680×1050+0+0 (normal left inverted right x axis y axis) 473mm x 296mm
1680×1050      60.0*+
1600×1000      60.0
1280×1024      75.0     60.0
1440×900       59.9
1280×960       60.0
1152×864       75.0
1152×720       60.0
1024×768       75.1     60.0
832×624        74.6
800×600        75.0     60.3
720×480        59.9
640×480        75.0     60.0
720×400        70.1

After that with a simple command you can use your dual monitor.

xrandr –output HDMI-0 –left-of VGA-0

Unfortunately it won’t work after reboot. To make it work after restart automaticly in fedora:

sudo vi /etc/rc.d/rc5.d/S25dualmonitor.sh

#!/bin/bash
output HDMI-0 –left-of VGA-0

exit vi

sudo chmod +x /etc/rc.d/rc5.d/S25dualmonitor.sh

Shortly, “S” in the name of file, means this file should be executed in boot process. 25 is order of process. For more:

http://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.html

PS: I’m using XFCE. In other desktop environment xrandr may ouput an error about resolition. To solve it, make sure your graphic card driver installed correctly