Great question — answering as the UphillTech team:
In my experience, most standard single sleeping bags, are built so you can zipper them together to make a double. They are designed to zipper to an identical/matched bag, so don't count on buying to different bags and have them work together. Instead of buying a dedicated double bag, buy a matched pair that can be joined. Google sleeping bags that can be joined together or sleeping bags that can be zipped together When you are sleeping alone, no problem. When you are sleeping together no problem. When you are in trouble and she makes you sleep alone, no problem.
Full answer by James Jenkins: https://outdoors.stackexchange.com/a/22906 (CC BY-SA 4.0).
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