could be getting microstutter as well. try enabling triple buffering to see if that helps.
could be cpu throttling (easy to check cpu usage)
could be texture swapping slowing the gpu's down if the scenes are complex enough... (you need a tool to measure how much memory is in use on the gpu's over time)...
PS: For the record I don't remember the witcher needing major amounts of cpu horsepower... what cpu are you using to power those 660ti's ?
PS: Try googling The Witcher SLI problems... I came up with several relevant hits
I have an AMD FX-8320. I'm not really seeing any stuttering or slowdown issues, I just had my display up at the beginning of the game to see how it was performing. During the part where they are walking into the castle, the camera pulls way back so you can see the whole area (hills, etc.) and at that point it dropped to around 50 fps. A couple of other times as well.
Maybe I should have been a little more specific - it's not performing badly in any way, I would just expect I could play it at a solid 60 fps with my setup.