I have similar setup with two 1600x1200 monitors (dell D810) and docking station one DVI and one VGA port with DELL 2001FP monitors connected.
have added the virtual settings into a basic xorg.conf and the xrandr screen shows the total size I set it to 3200 x 1200 on screen 0 It also shows three monitors DVI-0 (monitor connected to DVI port), VGA-0 (connected to VGA port) and LVDS (laptop screen)
So far so good. Now I am trying to understand this /home/username/.config/monitors.xml file I have been able to display the same screen on both monitors but I want it to span across both monitors. I am using the default RADEON driver delivered with Fedora 10. The monitors.xml file has the <clone>no</clone> set. How do you get this done with the new setup. I am close but no cigar yet.