My Uncle and his wife have a shared profile, largely because they only go on to see how I and my cousins are doing about twice a year, and don’t fully understand how to use Facebook. I think that’s perfectly healthy. This, however, is a fail. Plus the couple look around 15.
If combining fbs is the only thing saving their relationship then I would think it wouldn’t be worth it in the first place. If he cheated on her in the past, or vice versa, then they should just cut ties and move on. Insecurity is not flattering for anyone.