@Troll Nr 007, this is exactly the problem with my parents (Mirrors). They get along tolerably well with each other when it is just the two of them, but the introduction of a third party to the couple exposes their differences and probably makes them feel like their partner isn't as "on board" as they should be. That third party could be friends, acquaintances, or children.
Can you guess how many friends my parents have? Can you guess how much time they spent with their children?
When I visit them, it is like watching two lifers in a jail cell.