I have implemented your suggestions and here is Version 2:https://www.circuitlab.com/circuit/tgqxnr/unnamed-circuit-v2/https://www.circuitlab.com/circuit/tgqxnr/screenshot/1024x768/
An optocoupler such as 4n35 is sufficient replacement for your relays. My guess is they are cheaper and less bulky, not to mention silent. They can also be driven by the Arduino board so you don't need 9V (battery ground should be connected to Arduino ground).
I switched out the relays with a quad optoisolator (CNY74-4H) instead of the 4n35. Would this work?
Data Sheet http://www.mouser.com/ds/2/427/83526-97631.pdf
Just a practical advice. I would not let the camera focus for each shot because one lens may hunt for focus longer than the other and you most likely won't get simultaneous shots.
Set the focus to manual and you need to trigger only the shutter.
I already planned on using manual focus. The original schematic has an option to disable it. I plan to modify the code to use the button to focus the cameras at once before shooting and not in connection with the shutter.
I'm also not too sure about connecting the shutter pins together. I'd avoid it, if for no other reason, than just to keep my mind at ease.
I changed the schematic so the shutters & focuses are coupled after the optoisolator.
Did you have some other solution in mind?
Thank you for all the help.
Side note: My understanding is a optoisolator is basically the interaction between a LED & a photoresister. Something similar to a LA-2A Compressor?