I met this photographer at the re-enactment, also. I questioned him about the stereo camera. He told me that Matthew Brady sent out photographers to take photos of the participants of the Civil War. The intent was not to document the war (which they did), but to make money. Brady told his photographers to take 2 shots of everything, so that they were sure to get the picture. This photographer said the easiest way to accomplish that was with a stereo camera, so many used them.
This re-enactor Brady was taking photos of participants around the re-enactment. When pressed about the camera, he told me that inside were 2 Canon digital cameras. ;-)