I mean people who have been together for at least like 5 years. Most have been together more like 10 or even 40 or 50.
Most I know have been either duals or mirrors, but I wouldn't be surprised if I know as many mirror long-term couples as duals.
My theory is that these couples do well if they have an outside source who is the dual of one of them and activity partner of the other. Like, romantic couples don't have to give each other everything if the partners are getting those things elsewhere. My mother-in-law's sister is her dual. My father-in-law is her mirror. They seem to get along quiet well. That's an example.
And I suppose that could pertain to other relationships between romantic couples. Like, if they don't cause too much conflict with each other and they're getting their functional needs met by friends and family, it seems like they should be able to be quite happy in a relationship other than duality. Activity partners, semi-duals, etc., particularly. My parents are a look-alike couple and they've had problems over the years, but they've kept it together and are generally happy. My dad's best friend is his dual. My mom seems to have a few sources for Fi particularly - my brother is her dual, and my sister and I both have strong Fi. I don't know if she had a source before we were big enough to be of help. But then my parents had especially big problems before we were teenagers. It settled down quite a bit at that point. Maybe that isn't coincidental.
So, what relationships do you see in the long-term relationships around you?