For a residential boiler, all that's needed is to wire the two relay NC contacts in series with the thermostat. The boiler will shut off when either (or both) detector(s) is activated, and resume when they're both clear.
If you'd like a manual reset, install a start-stop pushbutton station & seal-in relay, and wire the two relay NC contacts in series with the stop button. Now the boiler will shut off when either detector is activated and remain off they're both clear and someone pushes the start button. Wired this way, the boiler will also shut off during a power failure and remain off after the power's restored.