While in some demanding games you might get close to 200% (i.e Metro). performance on average runs about 150 - 175 % of a single card in SLI....bit less w/ CF.....For example, In Metro 2033......however the dual GPU cards (6990 / 590) are abysmal in S.LI / CF
570 goes from 27 fps to 54 fps in SLI
580 goes from 33 fps to 65 fps in SLI
Here's some data using the games in Guru3D's test suite (COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead).
6850 - 170.89%
6870 - 161.52%
6950 - 156.78%
6970 - 156.81%
6990 - 118.50%
560 - 174.07%
570 - 166.60%
580 - 154.71%
590 - 111.46%