This Manfrotto device below isn't so much a single tripod head for 2 cameras, but a crossbar to mount multiple tripod heads or (camera ball mount style) heads --> two tripod heads, and two ball heads on the side for a total of 4 heads.. It doesn't make sense from a 3D , because the tripod heads will be too far away. However, you can shoot with sort of a laptop plate on one of the heads, and a camera right next to it.
You could get the Indieslider-Mini which is 2' long and ask for a second sliding camera mount on it. ITs quite light and you could probably even cut it down if you want.