How to wire 12 cameras to trigger simultaneously
If they're all at 3.6 V, you should be able to short all pins 2 together. Then short all pins 1 together. Then put one switch that shorts all pins 2 to pins 1. This assumes there's no safety issue with this, and all of the cameras operate at the same ground voltage.

To ensure there isn't some compatibility problem and you don't damage them, I'd wire up a BJT or a MOSFET to each camera. Then send one control signal to all of the BJTs or MOSFETs. If you want to go low-tech or you want isolation between the cameras, you could use 12 relays instead of the BJTs or MOSFETs.



EDIT: After reading the manual on it, it looks like they're all battery powered cameras and the GPI is just an input pin to a microcontroller that has a pull-up resistor on it (educated guess). You should be completely safe with my first suggestion, wire all Pins 1 (Gnd) together. Wire all pins 2 together. That will connect all of the pull-ups together. Then use one BJT or MOSFET to connect these two together if you want an electronic signal triggering them all. If you just want a push button switch, you don't need any electronics at all. Just wire them all together as suggested and then add a momentary push switch between the set of Pins 2 and the set of Gnd's (Pins 1).