Use up to three cameras and/or accessories at one time with Slik's Twin Pan Camera Platform.  It mounts to any tripod legset with a 1/4"-20 threaded post (this has a 1/4"-20 hole, not a 3/8"-16 hole), and has two panheads which allow photographs to be taken independently at any angle as well as a third 1/4"-20 on one side which you could use to mount a small tripod head or another camera.

You can remove one of the pan heads to use this device as a lateral side arm, and in that regard it has a maximum lateral displacement of ~13.8" (35 cm).

Twin pan/tilt camera platforms with spring-loaded 1/4"-20 camera screws

Camera mounting platforms can rotate camera while pan/tilt axis is locked

Bottom has a female 1/4"-20 threaded socket