What is the word or words to describe a relationship between two persons who kow each other very well without making any effort to speak things out and that they just know that the other one holds the same thought as he/she does?
Two people who understand each other with few or no words could be said to be in tune with each other.
in a state in which people agree with or understand one another · The president and his followers were clearly in tune. —usually + with · The new supervisor is not in tune with the needs of the staff.
Or they could be described as being on the same wavelength.
(of two or more people) to think in a similar way and to understand each other well:
What makes the problem worse is that Howard and Tina are not on the same wavelength about how to deal with it.
But if you're talking about the overall relationship, then they might have an intuitive relationship.
1 a : known or perceived by intuition : directly apprehended · had an intuitive awareness of his sister's feelings