The lenses they use to shoot in 3D might give someone the illusion it's two cameras side by side.
http://www.stereofinland.com/the-hobbit-3d-production-diary/
The side view of the camera clearly shows one unit behind the lens though.
Shooting a movie twice wasn't unheard of in the early 50's when it was uncertain if new costly widescreen processes would catch on. But this actually entailed shooting every scene twice, and resetting for each take with the other camera, not having two cameras side by side. The "flat" versions of these films are sometimes harder to see today than the scope version.