For relationships to be compatible at a close psychological distance, some traits need to be shared by both partners ("resonatory" traits), while other traits need to complement each other ("complementary" traits).
There has always been a debate about whether "like attracts like" or "opposites attract." The fact that this debate exists at all and has not yet been resolved is evidence that there is significant truth in both statements.
For the greatest compatibility, partners need to naturally strive to take responsibility for decision-making in different areas of life. Leadership should smoothly and painlessly change hands from one partner to the other depending on which aspect of their activities is at the forefront.
According to socionics, socionic duals are the most compatible types for long-term close relationships due to the nature of their information interchange, which satisfies the unconscious expectations of partners' psyches.
[Duals tend] to strive to take control over different aspects of activities. Each person wants to have his say and dictate certain conditions to others in certain areas. In other areas people are willing to give in and adapt to others' demands. If partners strive to dictate their rules in the same areas, they will inevitably have conflicts and feel unneeded by the other. By claiming leadership in a certain area, one essentially implies that others must be followers. People of the same socionic type rarely feel like giving up leadership of their favorite areas and forcing themselves to complement the other's behavior.
For duals, however, the process of transferring leadership takes place smoothly and naturally, as long as neither partner is psychologically damaged (I am talking about cases where for some reason a person relentlessly demands leadership in areas they are weak in).
This is because duals' zones of leadership complement each other. For example, if both partners are into sports, one will want to take responsibility for ethical aspects and the other for logical aspects. One will want to be the intuitive leader, and the other the sensing leader. This is quite an abstraction, but in practice the pattern is not too hard to recognize.