Created attachment 54399 [details]
xorg.conf Xorg.0.log pciconf -v output

This is with FreeBSD 8.2 Release, git version of the xf86-video-ati drivers as well as 6.14.2 and 6.14.3 versions of the driver. 

Setup:
  Dell 990 with 2 PCIE slots.    First card in "blue" x16 slot.   Second card in "black" x16 length slot ... As I understand, this slot is only wired for x4.

OS: FreeBSD 8.2 Release  -- GENERIC kernel

Screen 0 is on 1st card
Screen 1 is on 2nd card

after X is started, use xrandr to "expand" onto the second monitor of each pair using a command like this:
  xrandr -display :0.1 --output DVI-0 --primary --output DVI-1 --right-of DVI0
  xrandr -display :0.0 --output DVI-0 --primary --output DVI-1 --right-of DVI0

Display :0.0   is now spanned across the two monitors connected to the first card.

Display :0.1   is the 2 monitors connected to the second card.    First screen looks normal.   Second screen appears to be an offset mirror of first screen with corruption along one side.   (where the corruption appears depends on what resolution is brought up).

Have tried swapping monitors, split-out cables, swapping cards between the 2 slots, corruption always occurs on the second display connected to the second card.

If I reduce to only one card, I can always get 2 displays to work as expected.  Corruption only occurs on card in the second slot of the machine when both cards are installed.

I have 5 boxes with this same configuration.  All show same symptom.