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.