There are way, way easier ways to do this, but they do cost money up front and you don’t get that satisfaction of creating your own expensive and unwieldy franken-device that performs an obscure function. It has to be said that their little dual-camera rig looks pretty cool and is not limited by splitting the light as a simple mirror setup would do. The single-lens solution I linked above works up to f/11, which is really unbelievably slow. Anyway, DIY types and experimental photographers might want to give both a try.
Dual-Camera Stereoscopy [Patenteux.com, via MAKE:blog]