If I interpret what you are saying correctly, you will need to cut power to both live conductors?
In the case of 2 live conductors, both conductors should be switched and fused.
In this case you need to ideally set up a low voltage E-stop circuit and use this with a small control relay to switch off the coil to a power relay or contactor in the main supply that switches power to all but the low voltage control and PC. This way all your E-stop circuit is low voltage.
You could use the 12v power supply for this.
Ahead of all this should either be a double pole main disconnect switch, which it sounds like you have, or if close by, you can use the main wall plug and socket.
You can use the small relay on the arduino link to cut the coil power mentioned above.