Well there's actually three or 4 possible scenarios
The first is the one in your laptop, where the system switches between video cards as needed for better performance or power usage.Nvidia + intel setups use a switching technology called optimus. AMD systems use hybrid crossfire.
The second is SLI or crossfire where two cards work together.
The third is having multiple independant cards - this would work in older versions of windows if all the graphics cards used the same drivers, but having hetrogenous cards has been supported since vista. This is useful if you needed to add outputs.
Finally there's a GPU virtualisation technology called virtu which does a form of graphics card switching on desktops. This has more than one option - using the discrete or integrated card for output, and does about the same thing as the first option.