The expression “i due ragazzi” would be the same as “the two kids”.
The young couple would be translated as “la giovane coppia”.
In the case you mentioned you’re right, it just means that the two youths have no secrets to each other.
Without a specific context you can say nothing about the kind of romantic relationship, you only state that they are close friends.
Surely when you use the term ragazzi you usually refer to young people, but in general you can also use the term to ironically describe an older man acting as a younger one.
Suo nonno è proprio un ragazzo incorreggibile.