How to enable dual monitor with an ATI Radeon X300 card?

Question

I am trying to enable dual monitor for my Ubuntu 11.04 and ATI Radeon X300 video card. Here is the information about my monitors:

$ sudo lshw -C display
*-display:0             
   description: VGA compatible controller
   product: RV370 5B60 [Radeon X300 (PCIE)]
   vendor: ATI Technologies Inc
   physical id: 0
   bus info: pci@0000:01:00.0
   version: 00
   width: 64 bits
   clock: 33MHz
   capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
   configuration: driver=radeon latency=0
   resources: irq:47 memory:ec000000-edffffff memory:efde0000-efdeffff ioport:dc00(size=256) memory:efe00000-efe1ffff
*-display:1 UNCLAIMED
   description: Display controller
   product: RV370 [Radeon X300SE]
   vendor: ATI Technologies Inc
   physical id: 0.1
   bus info: pci@0000:01:00.1
   version: 00
   width: 64 bits
   clock: 33MHz
   capabilities: pm pciexpress bus_master cap_list
   configuration: latency=0
   resources: memory:efdf0000-efdfffff

And the video card

$ lspci -nn | grep VGA
01:00.0 VGA compatible controller [0300]: ATI Technologies Inc RV370 5B60 [Radeon X300     (PCIE)] [1002:5b60]

I tried to find documentation online but had no luck.

Answer 1

Set a larger maximum size for the virtual screen.

Create a file /etc/X11/xorg.conf if it doesn't already exist and add content

Section "Screen"
Identifier "Default Screen"
Device "Default Video Device"
SubSection "Display"
Virtual 2720 1024
EndSubSection
EndSection

In Virtual X Y, X and Y should be the screen size or larger than that appears in the error message.