I see this issue everywhere at forums (f.e. http://www.Tweakers.net ) that MultiGPU support is lacking and missing in the last patch.

Eighter for SLI double or tripple screens in SLI or flickering with ATi cards in CF, which makes the game unplayable. I can disable Crossfire as a workarround, but sad for the guys that are running with f.e. HD4850x2 as MultiGPU = 0 can not be used. Looking forward for that fix and any others that was reported from the MP Demo.

Good luck with the massive posts everywhere here.

http://blog.radeonpro.info/2011/03/crysis-2-flickering-on-crossfirex.html worked for me.