While ATI Technologies does not currently officially support any technologies that could enable multi-GPU graphics sub-systems on desktops or workstations, a recently released plan from Alienware is likely to provide users of ATI's RADEON products to use them in pairs to gain additional performance, similarly to NVIDIA's SLI technology.
Alienware, a leading manufacturer of high-performance computer systems, announced a licensing program for X2, a proprietary and patent pending technology for including multiple PCI Express x16 expansion slots on a single mainboard. This license is being offered through Alienware Labs Corporation, the research and development arm of Alienware Corporation.
Alienware's dual graphics cards option will not require driver support from the graphics card manufacturers. The system was designed to be standalone using Alienware's Video Array (including software and merger hub) and X2 mainboard. Video Array Technology divides the screen in multiple parts, in contrast to 3dfx's SLI that required every single line to be rendered by different graphics card. In the case of using two video cards, the screen is divided, vertically, in two parts: one video card renders the upper section, and the second video card renders the lower section.
News source: X-bit labs