Get a matched set and you can unzip completely and rezip together to double it. Most rectangular bags can do this, but not mummy bags (obviously). Or if you don't need serious warmth make your own with some sheets and light weight blankets stitched together properly. Much more comfortable in hot climes where you still want to be covered (ie, here in Florida where it can be mid 70s at night).
Shared sleeping bag does make tick checks a lot more fun
