3D images are not possible since the two cameras have two different (fixed) focal lengths (for wide and tele shots). There were plans to add zoom lenses (which would make 3D images possible) but they couldn’t make it happen (mostly because of technical issues, it seems).

(Hm, well actually, technically 3D images are possible, just not because there are two cameras. Since rocks tend to not move, Curiosity could just snap one image, drive a bit and snap a second picture.)