Smart Shooter does allow multiple cameras to be triggered with a single button click. However when triggering cameras this way, it introduces a delay of at least 150 milliseconds between each camera. This delay is unavoidable when triggering a camera over the USB connection, as the software has to communicate with each camera in turn.
So to trigger multiple cameras simultaneously, they need to be triggered by a different mechanism. This is possible using a dedicated cable release connected directly to each camera, or alternatively some kind of remote trigger.
One such provider of camera triggering systems is Esper Design, with their product called Shutter Cell, see http://esperdesign.co.uk
Or alternatively Pocket Wizards also provide various products for triggering cameras, see http://www.pocketwizard.com
Of course, at the same time, the cameras can still be connected via USB to the computer, so that Smart Shooter can automatically download any new photos, and camera settings can be managed from a single place.