for dual stormcores, (quad esc) i would use the CAN bus. Use one single button, connect the CAN bus in between them using 3 of the 4 pins (5V,CANH, CANL) no gnd.
Then configure using the setup wizard, it will automatically find the other Stormcore on the CAN bus and config it as well in a master-slave fashion.
Then you will be able to turn on and shutdown both SC using a single button.
But you can use a loopkey method also.