The Slik Twin Camera Plate 300 allows the simultaneous use of two cameras (or any accessories mountable with a 1/4"-20 screw) atop one tripod.
It has a pair of 1/4"-20 camera screws, mounted in tracks around an off-center tripod socket. Keeping the mount for the tripod off-center allows easier balance when using a larger and smaller camera, but the movement in the tracks housing the camera screws can also help to both keep the weight perfectly distributed or avoid a physical interference between the two items.
One of the tracks holding the camera screws extends roughly 1-2.4" (25-60mm) from the central tripod socket, and the other has an extended range of 1-4.7" (25-120mm). The mounting mechanism for the tripod actually moves on this plate, instead of you having to turn the entire plate atop the tripod threading like a propeller.