Many, me included, seem to experience bad performance on ATI crossfire systems.

This is due to what AMD calls negative crossfire scaling, which simply means that the game will run worse with crossfire enabled.

There are two solutions for this problem:

1) Install the latest APP profiles. The thing with this solution is, that there aren't any APP profiles available so far that would include a profile for TW2. Nevertheless, AMD already confirmed that they're woking on it.

2) Disable crossfire for the time until the required profile comes out.
You can do this by unchecking the box in the "Performance" > "AMD CrossfireX Configuration" menu of the Catalyst Control Center. Click on Apply and wait for about 30 seconds (the screen might go black a couple of times). Check if the box is unchecked now. If it is, you successfully deactivated crossfire. You can reenable crossfire by checking the box and clicking Apply again.

I have a 2x HD4870 setup. With crossfire I had 10-25 fps. Without I now have 20-40 but mostly a stable 30 fps with medium to high setting.

The new profile should be out in 1-2 weeks.

There's also another option 3), see http://www.gog.com/en/forum/the_witcher_2/tips_for_those_using_radeon_crossfire_setups/post19 .