I still have, and use, a Marmot bag that is a single that unzips to expand to double size and combines with an undersheet to form a double. The undersheet provides zero insulation, which is the usual case anyway. I don't believe this model is in production anymore, but perhaps there is something similar. My bag is marvelously versatile....
I have used two sleeping bags zipped together for me and my partner in the past, but I prefer systems such as @hikermor recommends.
Think of it this way - if either of you wants to get up, you must unzip the middle of the sleeping bag and let all the cold air in to both of you.
Nemo makes bags designed to zip together. Don't know how well it works but have seen it on their website.
Two square bags of the same type that unzip on the side and bottom should zip together when laid out flat so that one bag is on the bottom and one on the top.