You are on to the right idea but the wrong product. In an emergency, you can use a shipping warmer pack and tape it onto the side but don't put it inside the tub with the bp. Ideally, the best situation is a backup generator. You can get a cheap one at Harbor Freight for $80 that will run your fridge/freezer and all of your animal heat during an outage (for the 2 snakes).
I had all of our animals in their own outdoor building that's insulated and has a more than capable 5K watt heater. We had a freak cold snap last winter and it got down to negative 15 degrees overnight and the interior of the building dropped to 38 degrees! I added multiple space heaters directly in front of our snake racks and threw multiple shipping warmers into the dubia colonies. This was in addition to the 5k watt heater running full blast.
All of the animals pulled through fine with absolutely no ill effects from the cold night. I attribute this to also using high quality, enclosed racks and high quality heat tape that kept the tubs well heated even though the building got cold. It was a major lesson though. What if we had lost power? I have a whole house generator but it's not powerful enough to run the 12k+ watts continuously that the animal building drew that night... we could have lost everyone. By this Fall, I'll have moved all of the bps into the basement where it stays a fairly constant temperature year round. During the cold snap, it dropped down to 50 in our house but walking down into the basement felt actually felt warm. Our generator will have no problem powering their heat now during an outage no matter how cold it gets. As a bonus, the humidity down there is awesomely high for them.
So that's my advice for your situation (shipping warmers and a small, cheap generator) and then also my plan for ours based on recent events. Great thread and foresight!