Ritwika Mitra shows how to build your own dual camera mount featuring an adjustable interaxial distance for 3d stereoscopic photography or video. Ritwika is a high school senior, by the way. Cool young woman! She and her sister are also the founders of the non-profit Rennow.org.
The details begin at about 2:30 into the video. I built this but I also use a fixed mounting bracket that is simpler and smaller. At some point, I will post photos of my set up for 3D.