For NVIDIA SLI on Linux it's just a matter of specifying the SLI mode to use (Auto, SLI, AFR, AA, or AFRofAA) using the nvidia-xconfig utility or manually writing the option to the xorg.conf. Once that has been done and X restarted, Scalable Link Interface will be used where applicable. Through nvidia-settings, the SLI HUD (Heads-Up Display) can be enabled to visually show the rendering workload split between the NVIDIA GPUs. However, for most OpenGL programs the following string must be set before launching the program "__GL_DOOM3=1", otherwise SLI will not be enabled if the __GL_DOOM3 variable is not set.
On the ATI side, right now the setup is not as simple, but using CrossFire is straightforward. As of the Catalyst 8.8 Linux driver (fglrx 8.52), CrossFire is only supported on the Radeon HD 4850, Radeon HD 4870, and Radeon HD 4870 X2 graphics cards. Another caveat is that right now only two GPUs are supported by CrossFire on Linux. The four-card support may or may not come in the near future depending upon consumer feedback. The mix-and-match aspect of CrossFireX is supported on Linux to run say a Radeon HD 4850 and Radeon HD 4870 in a CrossFire configuration, but we have been told that it's not well tested at this time.
After you have installed the Catalyst 8.8 driver on a system that meets the criteria for CrossFire usage, the feature can then be enabled. The first step in the process is to create a CrossFire chain. A CrossFire chain consists of specifying the master graphics card and then the slave adapters. Right now with only two GPUs being supported, there is just the support for one master and one slave. To add a CrossFire chain, run aticonfig --adapter=0,1 --cfa where 0 would be the master graphics card and 1 would be the slave. To find out the order of the graphics cards, run aticonfig --lscc to see all ATI graphics cards capable of running CrossFire. If all went well, the CrossFire chain will have been created and its configuration written to the AMDPCSDB, or the AMD Persistent Configuration Store Data-Base.