Actually, and based on my knowledge, exposing the part to that temperatures may not cause a big damage, or apparently will not shorten its lifetime. But it wasn´t characterized in that conditions, so ADI cannot guaranteee that no damage will occur. So I think it could be a good idea to go ahead with the shutdown circuit.
If you finally decide so, how about using the interruption output on ADT7410 (setting the low limit to -40ºC) to act over a switch that will manage the ADT7420 supply?
http://www.analog.com/en/switchesmultiplexers/products/index.html
Regards,
Lluis.