In general, the answer is if you have two PCIe x16 slots, you can use any two graphics cards you want. Whether or not the OS will make use of it depends on driver support. They will not compliment each other in terms of performance, i.e., they won't both be used in a game, except you buy two of the same card and use SLI or Crossfire, but even then it's a crapshoot if it'll work or provide benefit.
The most common use case for having just another video card in the computer is to add display outputs.