The theme of judging allows the Savior to discuss the position of these twelve as judges in Israel. In particular, they are the judges of the New World branch of the house of Israel. Tersely, the Savior reemphasizes themes from the Sermon on the Mount. By these slight references, he expects that the twelve will understand the context, and recall the greater detail from the Sermon. The first reference is the more condensed of the two. The Savior tells them to judge, but to use the Savior as the model for judging. This is the theme from 3 Nephi 14:1-2, where the Savior discusses the issue of judging (“judge not that ye be not judged…” See the commentary following these verses).
The second reference is more direct, and refers the twelve to 3 Nephi 12:48 where this command was given in the Sermon.