AMD's driver installation either goes just as smoothly as Nvidia's or get ornery. The best-case scenario is that you plug the graphics cards into the mainboard, connect them with a CrossFire bridge, start the system, and install the driver. The worst-case scenario is that cards aren’t recognized by the drivers. If you drop back to only having one card inserted, the driver will install, but as soon as the second card is plugged in, the Catalyst Control Center (AMD's graphics driver interface) returns error messages complaining about missing files, preventing access to the CrossFire settings.
If you run into these issues, you can typically solve them by first installing the main graphics card, and then inserting the second card. If you still can't proceed, remove the CrossFire bridge until the second card is detected correctly in the device manager and all drivers are installed. If this still does not work, you can try installing the first card in your primary PCIe slot, remove it, install the second card in the secondary PCIe slot, and then finally insert the first card in slot one again.
AMD's drivers can be fussy. Even if the driver successfully detects the graphics card, it'll sometimes misbehave if you then try to insert the card into a different PCIe slot. If you install the cards separately, try inserting both and then connect the CrossFire bridge. If you're still having problems, another driver is needed, because uninstalling and reinstalling doesn't always work, either. In our testing, the easiest way to do this turned out to be using the (officially unsupported) drivers from the preview package. Hopefully, this will finally let you use the Catalyst Control Center, because this graphical interface is required to activate CrossFire.
Once CrossFire-compatible cards are installed and the drivers work, CrossFire activation will be suggested. The activation check box is almost always checked by default, but you should still adjust anti-aliasing and texture filtering to taste, and then reboot so that all settings are stored and active. With current AMD drivers, you can't activate CrossFire without a CrossFire bridge physically connecting the graphics cards (unless you're using certain mainstream configurations, which do support operation without bridges). On the bright side, you can use CrossFire with different classes of GPUs. However, performance scaling will be imperfect. Don't expect to get more than twice the performance of the weakest card.