Sometimes the original text uses the historically singular pronoun thou in a plural context. In this passage the angel is speaking to both Laman and Lemuel:
Then after the single use of thou (“behold thou shalt go up to Jerusalem again”), the remainder of the verse continues with the historically plural pronoun (“and the Lord will deliver Laban into your hands”).
All such examples of thou being used in the plural have been systematically edited out of the Book of Mormon. (For another example, see the discussion under 1 Nephi 7:8.) Although mixtures of ye and thou can occur in passages referring to one person (see for example Alma 37:37), their use within the same clause has been eliminated. For a complete discussion, see THOU in volume 3.
Summary: The original text allows mixed uses of thou and ye within the same clause; in addition, thou occasionally refers to more than one person; both of these infelicitous types of usage have been edited out of the text but will be restored in the critical text.