GoPro Dual HERO 3D stereo system only compatible with HERO3+ Black Edition units?
I probably could have done a quick google search before asking. HERO3+ Black Edition may be the only currently selling supported unit, but I found that following historical products are compatible with a "3D HERO System" (which may be different from "Dual HERO System"):


HD HERO Original camera
HD HERO2 camera


Dont know whether "3D HERO System" has different sync cable than current DUAL System. Also, a guy called "linuxluver" said on this page that he is using his "Hero3 Silver cams for 3D content". I am not sure if he uses the sync cable or not.

Still, they list the HERO3+ BE as the only compatible with "Dual HERO System".