Using the ALE as a determination if the controller has failed is probably not a useful. So how do you determine your controller has failed? Do you detect if the thermocouple has failed? Do you detect if the relay, solid state relay or whatever means you use to control the temperature fails? Detecting if the microcontroller has failed is reasonably easy - have a little charge pump circuit that needs its input to be toggled regularly and coupled to a relay. If the micro doesn't toggle the port pin, the relay drops out and switches over to the other controller.