I invested in a generator, the peace of mind is great, i highly recommend it. Plus it can be good for people too LOL.
For short durations i also wrap tanks in a blanket (take some yarn or rope and secure blanket/s in place. and add in warm water on a regular basis. But that does not work well long term. I would think you might be able to also help with temps by opening a door under the cabinet and putting a kerosene heater infront of the open door, the heat might rise through the bottom to help keep it warm ,be careful of any blankets covering the tank, be sure to move them securely out of the way. You might be able to tent the back side a little using hooks or a nail so the warm air could fill it up and help maintain a warmer temp like a cocoon. I would still add in fresh warm water too since the oxygen will be depleted over time. Your best bet would be to use a generator though. Maybe you can transfer some fish (becareful not to overload the tank) to the warmer tank to ensure some fish from your second tank also survive.