If one is using DSLRs and OK with a few hundred millisecond delay between "simultaneous" exposures on 2 cameras, then
DigiCamControl has a multiple camera feature that can be used to control exposure, etc, on multiple cameras (I've yet to try its multiple camera feature and hopefully it offers the same degree of control as it does in the Astro module).
Not that you'd need it but, the literate suggests it could handle up to 127 cameras via USB and Hub, just not perfectly simultaneously (a USB communication, not software limitation). There would be a 0.2 to 0.8 second delay between cameras (Not an issue for astro if organised appropriately for things like dithering).
For info:
GUI ....
http://www.digicamcontrol.com/doc/us...rface/multicam
Literature....
http://www.digicamcontrol.com/doc/usecases/multi
DigiCamControl software handles, Canon, Nikon, Sony and many others and incorporates an astro specific module amongst other features.
Best
JA