Automatic transfer switches are a thing, but generally you want the supplies sync’ed. A manual break-before-make transfer switch will do the job. Not much help

Automatic transfer switches are a thing, but generally you want the supplies sync’ed. A manual break-before-make transfer switch will do the job. Not much help if you’re not home and the mains goes out and your food spoils, though fridges will stay cold for hours if left shut, and longer if there’s a lot of thermal mass in them - try to keep most of the empty space in your fridge and freezer filled with water bottles.