I'm guessing killzone 3d renders both the left eye perspective and the right eye perspective simultaneously. 2 full renders. Thus, this is why they take a strong hit on resolution and frame rate.
Yes, that is most likely correct.
It's also the Right Thing to do as far as stereo 3D.
My guess/understanding.....
Unfortunately you're probably wrong with your idea.
What Crysis 2 seems to be doing is to render a single frame with no stereoscopy or anything extra at all, just your every day rendering. It also keeps the Z-buffer. Hence the lack of performance penalty.
Now for each pixel you have a Z value, which is the depth information - how far it is from the screen's plane. They also have to calculate a relative distance for the eyes' viewpoints (sort of an ideal viewing position, ie. X feet away from an N inch screen).
Then they begin a simple 2D image processing pass, moving pixels to the left/right to create the left/right eye's images. Using the Z-value and the above mentioned viewing distance, it takes very simple math to calculate how much you have to move the current pixel.
As phil has mentioned, the trouble here is that you don't have any image information for the holes that you create in the image when you move these pixels around. The enemy has covered up the wheel of the car, now he's moved and you'd need to see that wheel, but there's nothing to work from.
This is the 'magic' part, coming up with something to patch these empty parts of the final image. Having no extra information but the base image, they probably have to resort to stuff like what Photoshop's "healing brush" does, or probably something even more simple.
When you convert a movie from 2D to 3D like this (Alice from Disney) you have all the time in the world and can take some operator guy to paint something into these empty areas. There's even some significant research in this field, although using an algorithm to fill up holes like this in a sequence of images requires a high level of consistency and determinism.
In the end, it will never be as good as actually rendering from two points of view.