This is a good time to put all of your video cards into your system and set up your monitors. Set everything up the way you want it when you are done, as you will have to repeat steps later if you change things.
This next step needs to be done from the console with-out X running. If you are in X, exit now. If your system uses a display manager such as xdm or gdm exit you need to stop that service. If you need to stop a display manager from RedHat the easiest way is like this:
root# > telinit 3
for Debian, Storm or Corel :
root# > /etc/init.d/xdm stop
If neither of these methods work you, reboot your computer and start up in single user mode.
In a multi-head setup you need to explicitly identify each video card in your config file. To do this you need to use the PCI Bus Identifier your system assigns the card. At this time, all video cards need to be in your system.
To find out what your PCI bus IDs are:
root# > XFree86 -scanpci
X will then output a code for each device on your PCI bus.
(0:0:0) Intel 82443BX Host
(0:1:0) Intel 82443BX AGP
(0:7:0) Intel 82371AB PIIX4 ISA
(0:7:1) Intel 82371AB PIIX4 IDE
(0:7:2) Intel 82371AB PIIX4 USB
(0:7:3) Intel 82371AB PIIX4 ACPI
(0:9:0) Adaptec 2940U/UW
(0:11:0) 3COM 3C900 10b Combo
(0:12:0) Matrox unknown card (0x2007) using a Matrox MGA 2164W
(1:0:0) unknown card (0x1092/0x0152) using a Texas Instruments Permedia 2
PCI cards and devices are identified with a leading 0, AGP cards with a 1. In my case I have a Matrox Millenium II (0:12:0) and a Diamond FireGL(1:0:0). Find your video cards and write down the numbers associated with them. It is important that you know which number goes with which card.