Normally you'd just have to list the devices in your /etc/X11/xorg.conf. For example in my xorg.conf I have the following three devices:
Section "Device"
Identifier "aticonfig-Device[0]-0"
Driver "fglrx"
BusID "PCI:1:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[1]-0"
Driver "fglrx"
BusID "PCI:4:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[2]-0"
Driver "fglrx"
BusID "PCI:5:0:0"
EndSection
You can find the BusID by running the lspci command and looking for your graphics devices. When you add your devices to the xorg.conf they should be running after you restart your x server.
Cheers,
Dominic