Should work, but I can't test atm, not at my sim machine. You just need to replace XYZ and ABC with the button numbers of your cutoff switches, which can be determined by running the "Show joystick button numbers"-script from FlyWithLua's menu.
-- Engine 1 cutoff switch
create_switch(XYZ, "sim/flightmodel/engine/ENGN_mixt", 0, 1, 0)
-- Engine 2 cutoff switch
create_switch(ABC, "sim/flightmodel/engine/ENGN_mixt", 1, 1, 0)
The logic behind that command is (button number, DataRef name, array index, value when button is released, value when button is pressed)