The Gitzo G-1539 is a Heavy Duty Double Camera Platform that allows two cameras or two heads to be mounted to a tripod column. Suitable for 35mm or 6 x 4.5 cameras. Measures 17" (43 cm).
Supplied with two 3/8" male screws, and use of a ball head at each end is recommended.
I bought this to mount a dual camera system (pair of Canon T3i's) for doing 3D Stereo photography and maybe video. Overall it is working out great. So far the only real issue is the knob holding the bar onto the tripod seems to loosen up too easily. it also seems almost as heavy as the tripod itself. When I initially got it the bolts to secure tripod heads to the mount did not fit the camera's lock points, but this is a minor point since I had intended to get and have gotten tripod heads since then, which make it possible to configure the cameras for portrait mode and get them a touch closer together. For 3D stereo, it looks like the mount allows for good focal plane separation for stereo shots out to about 150 feet. I have also amazingly been able to hand hold the mount allowing for some unique angles I would have trouble getting with just the tripod. In short, very glad I bought this one.
Bought this to avoid having two tripods when travelling and taking long exposures. Turns out having two cameras on one tripod worked against my standard long exposure habits and I returned it for that reason.