On Red Hat, go to
Main Menu ->
System Settings ->
If you want to enable the 2nd monitor, check the box next to
Use Dual Head (depending on the version of Red Hat/Linux, there might be a separate tab in the
Display dialog window for Dual Head graphics options). To change the 2nd monitor's settings, click
According to this article, Configuring the system for the second monitor on Red Hat Enterprise Linux 4 and Fedora Core 4, you can enable both monitors to use an "extended desktop" by:
Desktop layout option to Spanning Desktops. To create a pair of
independent workspaces, set the Desktop layout option to Individual
Desktops. With "spanned desktops" you can drag windows from one
monitor to another; with individual desktops only the mouse cursor can
move between desktops.
Note that you have to save your settings and then log out and back in (or reboot) your computer in order to see the changes, they won't happen right away.