I have what I think is the best for this, although not perfect. It is a WM Badger. Very warm, big enough that it works for two. One can still use it as a great bag alone. I wish there was a way to get rid of the hood, like a zip of version. Still, it works great. I pin a diomond shapped piece of Epic fabric under it. This keeps out drafts and gives us something to lay on too. Not necessary but nice. I then unpin it when I use the bag alone. Half the time we just drape the thing over us. We have never been cold.
Yes, I am selling but only b/c I'm broke, not out of a lack of love