For example, a system with a GeForce GTX 280 (OpenGL 3) and a GeForce GTX 580 (OpenGL 4) can do tessellation (OSX 10.9 Mavericks) only in the virtual screen of the GTX 580. Setting the virtual screen can be useful in some cases. OS X OpenGL framework provides some functions to know the number of virtual screens, to get and set the current virtual screen ( CGLGetVirtualScreen and CGLSetVirtualScreen). Depending on the position on the screen of the OpenGL application, OS X dynamically uses the right OpenGL driver plugin (NVIDIA, AMD, Intel or software renderer) and implicitly updates the rendering context.

All the work (rendering context management) is done by Apple in its OpenGL framework. You have nothing to do to manage virtual screens. If you connect a monitor to each card, you will have two virtual screens. With OS X, you can have several different graphics cards, for example a Radeon HD 7970 and a GeForce GTX 580. The great thing with virtual screens is that an OpenGL application can run at full speed in a virtual screen. Then if you have two graphics cards and two monitors, you have two virtual screens. Under OS X, a renderer can be hardware (a GPU) or software (APPLE Software Renderer). A virtual screen is the combination of a renderer and a physical monitor. Under OS X, all these situations are not problematic thanks to virtual screens. And if you have a GeForce card and a Radeon card on the same motherboard, there is nothing to do. You can’t do SLI with a GTX 780 and a GTX 460, same thing for CrossFire: coupling a HD 7970 and a HD 5770 is not possible. What’s more, SLI and CrossFire require nearly identical graphics cards.

Under Windows if you have two graphics cards, there is only one way to use both GPUs with OpenGL or Direct3D: NVIDIA SLI or AMD CrossFire.

I’m sure that GPU overclockers under Windows would love them. Virtual screens are a very nice feature of Mac OS X.

Mac os x mavericks opengl