Re: 2 camera's on one tripod
 

I own a 2' Kessler Pocket Dolly that I mount upside-down on a tripod half-ball (shuttle side down), then use two Kessler flat-mount pucks to attach heads to the underside of the slider. Can choose 1', 2', or 3' separation between the cameras depending upon your needs and slider version.

Of course this is a rather specific and not-too-inexpensive solution if you don't already own the elements, but does do a wonderful job of multi-purposing gear and weight.