These scenes are apart of a whole dramatic irony plot because of the lack of full awareness these two characters have over the whole situation. Chandler and Monica do not know that Rachael knows or that Joey knows that Rachael knows, but the audience do. Funnily enough, later on in the next few episodes, all other characters find out about their relationship unknowingly by the couple.
In the Show Friends, two of the five main characters began secretly seeing each other until one of the other friends Joey finally discovers their relationship. The couple then tell Joey not to tell anyone else until they are ready, so to them only Joey knows.
Rachael, another friend, ends up finding out about the relationship after accidentally over hearing them making plans through their home phone, Joey then tells her that he knew already, but that she is not supposed to know.
Dramatic irony is when the outside audience or reader are aware of something that has not been realized by the characters.