This can be achieved by connecting the same external trigger signal to one of the digital inputs of each camera like it's shown in the following figure:

Figure 1: Electrical setup for sync. cameras

Each camera then has to be configured for external trigger somehow like in the image below:

Figure 2: ImpactControlCenter - Setup for sync. cameras

This assumes that the image acquisition shall start with the rising edge of the trigger signal. Every camera must be configured like this. Each rising edge of the external trigger signal then will start the exposure of a new image at the same time on each camera. Every trigger signal that will occur during the exposure of an image will be silently discarded.