A professional photographer I know, who specialises in 3-D images, says that really effective 3-D photos need the lenses to be around 6 to 9 inches apart depending on the subject. This distance necessitates two seperate cameras. He says that the various dual-lense 3-D cameras (they've been made for film for many years as well as more recently for digital) just don't produce a truly effective 3-D effect.
Having helped him on a few shoots, the one feature that really helps the3-D effect is to have one (or more) longh, thin objects projecting relatively close to the camera - when you then use a steroscopic viewer, the object seems to 'pierce your eyeball' and massively emphasises the 3-D effect. You get the same effect when interpreting sterescopic vertical aerial photos when there are tall factory chimneys - they seem to stretch a thousand feet into the sky.
One of this photographer's specialities is 3-D oblique aerial photos. For those, the images have to be taken a few hundred feet apart, the distance depending on the altitude the plane is flown at. A 3-D aerial image of a large slate quarry would be amazing...