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Yes, the blue lines are the thermocouples which are the only sensors used by cheap thermostats. I just wanted it to be clear that all four were monitoring the same zone.
 
Hi folks
The lasting solution to the problem is controlling both thermostats with one DPDT relay/contactor.Both thermostats can have same maximum threshold but the minimum setpoint need to differ.
Connect two leads of the 'main thermostat' to two normally closed 'NC' contacts and connect the leads of the standby thermostat to two normally open 'NO' contacts(these two leads must be expended to the relay /contactor)
and connect the two 'Common'leads to heater.
Let us assume main thermostat has minimum set point of 100 fahrenheit ,the substitute must have a lower setpoint of say 99.5,99 or 98 deg fahrenheit and both have have maximum threshold of 110 deg farenheit
.When the oven is first switched on,
the normally closed contacts of the main thermostat controlling the temperature

is opened so that the standby one controls temperature for the first cycle until the maximum setpoint of say,110 is attained and then de-energises
and takes a 'holiday'.When temperature falls to minimum setpoint of
100 deg farenheit,the main thermostat starts doing it job until there the oven gets switched off/
there is power failure then the cycle starts all over again.
In the event the main thermostat develop a fault ,the deputy thermostat simply takes over the job of its late 'boss'.

This is similar to reversing a motor using DPDT relay or contactor.The circuit looks similar to the following one posted by Reloadron for reversing single phase motors:https://www.electro-tech-online.com/threads/3pdt-to-dpdt.119949/
 
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