There can be a slight weight savings for a double bag over two single bags. This is difficult to fully calculate, especially if one person sleeps warmer than the other or if there is a height difference (e.g., one person needs a long bag and the other a short bag). There can also be a comfort advantage, but it really depends on how narrow the double bag is. I have a wide single bag for car camping which probably qualifies as the "spooner's only fit" in your link which is no where near as comfortable as mated single bags, but definitely the lightest option. My real dislike of mated single bags are the middle zippers and separate foot boxes.
For long hikes there are a couple of serious disadvantages. First, a double bag means you must camp together every night. It also means you need to stay in closer contact on the trail as one person is not carrying, what is in my opinion, the proper emergency gear. Further, if the bag gets wet, there is no backup (and probably a lot of blame). I would also be worried about drying the bag out in town since getting a double bag in a dryer might be problematic. There is also the issue of weight/volume distribution.