Crossfire is an AMD thing. Doesn't work with Nvidia cards. Nvidia cards use SLi.
You still can't SLi, them, however. For Crossfire or SLi to work, the cards need to be identical, and even if it could work, the overhead for SLi would be greater than the power provided by a card as old as the 8600GT. Seriously, the 560 is waaaaaaaaaaaaaaaaaaaay more powerful. Trying to combine the two would be like trying to improve a 50-cal by duct-taping a potato-gun to it.