The judge may know they have been untruthful as in their statements are not consistant or they have discredited themselves on the stand. I belive for purjury you have to testify under oath one way and then under oath testify in contradiction to the first testimony. I'll have to look it up again.
It definately shows that someone coached the VD's in their testimony because in jury instructions it says to not judge testimony a person gives as false if they say that they do not have recollection of specific facts because memory of witnesses tends to blur after time.
So if I say well I really don't recall but I might have said this or that the jury can't fault any of my statements if it does come back that I did in fact state it. Most people unless represented by a lawyer would not testify as DVD and BVD did, hell it sounded like they were the defendents for cryin' out loud with all the "I can't recall" and "I don't recall saying that." To the most minor questions.