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 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.