Multi-Monitor Question

According to this site, it's heavily dependent on the OS, graphics card, graphics driver and API used (OpenGL/DirectX)...

OpenGL: http://www.realtimesoft.com/multimon/ogl/
More on OpenGL: http://www.realtimesoft.com/multimon/guide/multimon_opengl.asp

DirectX: http://www.realtimesoft.com/multimon/programming/3d_graphics.asp