If you need anything else let me know, I would like to set up another 6750 to hopefully push out better fps for mmo's and try to figure out how to OC them
Also hopefully this is in the correct spot, if not please feel free to move it where it should be.
You must first install the second card for the SLI/Crossfire option to be available.
It will then show up in the driver control panel in windows.
Unless you can get the second card very cheap, performance would not be worth it.
A new 7790/R9 260x would be about the same performance. Draw less power and not have the hassle of a dual card setup.
Crossfire of older cards is a gamble. If your game has good support you could get 50-70 percent improvement.
Titles with poor crossfire support can run slower under crossfire than with a single card.