Are you installing the driver and running aticonf --initial before changing the system configuration or after ? I think you need to run aticonf --initial after, but not 100% sure.
I am using the ati oss drivers. I haven't check this with the fg drivers yet. But I am pretty sure I would get this there too. the only thing i run X -configure
Note I'm not quite sure when the actual drivers get installed but, this error messages happens like write when you get past grub like immediately after. I suspected the video drivers got loaded a little later.
I only get this error when I enable surrondview in bios, to enable me use the integrated graphics + video card. And this message is in Kern.log file.
Sorry, your "crossfire" comment threw me off; the open source drivers don't support crossfire.
There's not a lot of work going into the open source drivers for multi-GPU configurations at all right now, at least not until RandR gets extended to handle more than one GPU and the acceleration code is rewritten to intercept and split drawing commands across multiple GPUs. The current RandR multihead model simply defines a large virtual buffer and positions screens within that buffer; great for multiple screens on a single GPU but doesn't scale to >1 GPU.
Your best bet for multi-GPU support with 6xx-and-up GPUs today would be fglrx, since it already includes all the functionality for splitting drawing commands across multiple GPUs and screens.