Puting 2 thermostats in series with both set the same will offer some over temperature protection. You may expect though that the average temperature may drop a little as both thermostats will need to kick on before heater will turn on.
Other trouble is although having two thermostats cuts the chance of an over temp condition in half it doubles the chance of an under temp failure as if either of the thermostats fail open you lose heat. Low temp is likely the lesser of two evils though.
Just a thought on your incubator, if it was able to reach 160f it means it is operating an a low duty cycle with long periods of element off time and short periods of intense heat. Set up like this the heater might be off 70% or more of the time and on 30% or less of the time. If the thermostat fails closed it can quickly overheat, also the on and off cycling is going to be frequent, causing temperatur fluctuations and a more uneven heat.
A light dimmer switch can be added to the circuit to cut some of the power. With less power availible to the element it may be possible to fine tune it such that the element is on 90-95% of the time and off only 5-10%. In this way there will be fewer temperature cycles and the temperature spikes caused by the element cycling will be less abrupt with less overshoot and less radiated heat energy so the eggs can stay a more constant temp. I am not sure if you are using a bulb or element to heat with but if it is run cooler for longer your temperature wil stay more constant as it heats the air slowly and creates less radiated heat that can heat the eggs unevenly.
Given that thermostats are typicall electro-mechanical devices and like all electro-mechanical things have a finite life. Reducing the number of times the unit needs to cycle and reducing the curent the device needs to pass will reduce the chance of the thermostat failing.
Adding a dimmer is one more potential problem though as it makes the whole unit more complex allowing for human error to sneak in, but I think it is managable by securing the dimmer so that it can't be accidently adjusted and cause insufficient heating. Also some care needs to be taken to ensure that if the surrounding air temperature drops say at night when the house cools off the dimmer still allows enough power to the element to keep temperature without hitting 100% duty cycle where the unit won't be able to hold 99.5 even running the element constantly.
I use a reptile t-stat and it must have gone out in the night. I had over 70 eggs that I had paid over $100 for! GRR! The ones that were due to hatch on Friday were leaking what looked like puss, the 30 silky eggs I just put in on Sat were soft boiled. 
