ATi's Radeon graphics cards also support multi-monitor operation, although this is limited to the Dual-View and Clone modes. The vertical and horizontal span modes required for many games are not supported. Multi-display configurations are managed using the HydraVision software that ATi offers as a separate download on its driver pages. Two versions of this software are available. The first is the basic, no-frills version that supports the basic settings as well as window management features. The extended version offers more window-management options than the basic version and also features an option to use several virtual desktops similar to those of the Unix/Linux X environment, which ATi creatively calls Multi-Desktop.
We're not sure why ATi's cards don't support spanning modes. Most likely, the driver is to blame. The driver treats ATi multi-monitor cards like two individual devices, each with its own driver. As a result, ATi's cards have to be considered second choice for dual-monitor gaming setups.