I've been waiting for answer to this for a long time,
I have similar setup currently on Asus Crosshair Formula V, with an HD 5970 and HD 5870 (all of which are the same GPU Cypress XT I believe). I have a 990FX Chipset with a FX-8150 driving the system.
I now the IOMMU has something to do with the issues but really what it comes down to is the drivers they release as this issue hasn't been addressed properly since 2008 lol
AMD needs to get more aggressive with their Linux support as Nvidia has been pretty good with theirs in which they also recently came out and said they will make it even better due to the the increased demand for Linux gaming. Game Developers like Blizzard and Valve as well as a few others are now ready to start working with the Linux Community for a number of reasons.
Windows 8 upset allot of people both end users and developers so that is one reason, another the Linux Community has been growing super fast I have never seen so many people aware of the OS and interested in running it. The next generations are allot more tech savvy. I see young teens running around with live distro's on their USB sticks constantly.
this question thread seems to be an answear to my own question:
Should Crossfire works on Linux (Fedora 18 64bits with 2X HD 5750)?
Because currently, my first card is running at 99% and the second one is sleeping (0% usage and slow frequency) under Furmark (windowed)? I checked it with "aticonfig --adapter=all --odgc --odgt".
But in fact I need it for a small openGL FBO program I have done to accelerate computing and it's working fine (and even great) but only on one card!
Is my only chance to get the 2 cards busy is to convert my program to openCL?
I know that openCL is working fine as I use luxmark bench successfully (1 GPU, 2 GPU, 2 GPU + 1 CPU).