Dave, looks like that was a documented issue in a previous driver release, though we've only just put the pieces together here. Looks like it was solved for DX10 and DX11, but not DX9. AA8 quoted from his thread on HardOCP:
This was at the time of 8.66 drivers, but I guess the 10.3 drivers don't fix crossfire in portrait mode yet
___________________________________
Radeon 5970 Eyefinity support
Right now ATI Eyefinity support with multiple displays is officially limited to single-GPU configs only. CrossFire doesn’t work just yet.
...
Now the Eyefinity support does come with a number of caveats. For starters, support is limited to the games listed above only. No other games are supported yet at this time. In addition, only landscape configurations scale properly. Running portrait mode will still function, but will rely on the primary GPU only. And again, Eyefinity support is limited to the 5970 card at this time.
http://www.firingsquad.com/hardware/...view/page3.asp