Download Nvidia inspector tool.

http://downloads.guru3d.com/NVIDIA-Inspector-1.94-download-2612.html

Search the profile for the Witcher 2.

Replace the default SLI compatibility bits with this custom one:

0x02C04205

I just tried it with the start of the campsite level which previously ran about 20-30fps now runs fluid 60fps with Vsync enabled

Noticed some lighting glitches in some cutscenes so the compatibility bits ain't perfect. But better than the default atm.

I have just gone through the prologue and did not perceive any problem with my SLI configuration (default settings).

I will love to know more about this issue with the SLI configuration

My setup is:

Intel i7 920
Win 7 Pro 64bits
12 GB Ram
GTX 460 720Mb (x 2 in SLI)
Resolution: 1920x1080
W2 Details level: Hi Spec