Sadly, a modern day french door is pretty inefficient. Consumers prefer them because of their looks but they are very far from being an efficient way to keep food cold.
Why are they any less efficient than a single-door fridge? Losses in between the two doors? I don't see why it should be much different. Both types of fridge causes the cold air to "fall out" when you open the door(s).
A top-loading chest fridge/freezer is of course most efficient, but don't think many people have those in their kitchens.
French door has less insulation because there's more "seams". Also typical french doors tend to have an ice machine or water dispenser, which is even less insulation and/or more seams.
