I don't think you need a double sleeping bag since most bags will mate with another bag of the same brand. About a thousand years ago, when I took trips with a girl friend, I had two North Face rectangular bags -- one with a left zipper and one with a right which zipped together to make a comfortably large "double" bag. This way, she still had to carry her own weight. Worked fine until I got married and my wife made me quit backpacking with my girlfriend.