The 1837 edition deleted the repeated their in “their great and their true shepherd”, probably accidentally (the change was not marked by Joseph Smith in the printer’s manuscript). Of course, normal English phraseology avoids such pronominal repetition when attributive adjectives are conjoined within a noun phrase, so the loss of the repeated their is quite natural.
There are a few cases in the text involving great where a determiner is repeated before conjoined adjectives within a noun phrase, but these examples are limited to the articles a and the:
Of course, the article does not have to be repeated, as in these examples from 1 Nephi:
More generally, however, there are no other examples where the determiner their is repeated for conjoined adjectives, nor is any other possessive pronoun (my, thy, his, her, its, our, or your) repeated in such an adjectival context. In fact, conjunctive adjectives are rare when the determiner is a possessive pronoun. Other than the example here in Helaman 15:13 (where the their is repeated in the earliest text), there is only this example:
Even here, initially in 𝓟 and in the 1840 edition, the expression “and your worthless and fallen state” was sufficiently unexpected that the adjective worthless was replaced with the noun worthlessness, thus giving two conjoined noun phrases (“and your worthlessness and fallen state”) rather than two conjoined adjectives (“and your worthless and fallen state”). In the original text, then, there are only two instances of conjoined adjectives where the determiner is a possessive pronoun: in one case, the possessive pronoun is repeated (here in Helaman 15:13); in the other, it is not (in Mosiah 4:5).
One wonders if the repeated their in Helaman 15:13 was accidentally added when the text was dictated. The repetition of their in the immediately preceding text (“the knowledge of their Redeemer and their great and …”) could have led to such an error. There is evidence elsewhere for the occasional addition of a repeated their:
But because there are only two instances of conjoined adjectives where the determiner is a possessive pronoun, there seems little motivation for making one of them agree with the other in either the repetition or the lack of repetition of the possessive pronoun. As noted above, there are examples of conjoined adjectives where the determiner is either the indefinite article a or the definite article the; and in those cases, sometimes the article is repeated, sometimes not. We have the same situation, although much less frequently, with conjoined adjectives when the determiner is a possessive pronoun. The critical text will therefore restore the repeated their in Helaman 15:13 since it is extant in 𝓞. And in this case, 𝓟 and the 1830 edition (both firsthand copies of 𝓞) agree with 𝓞, so all the early textual evidence consistently supports the possibility of repeating the their here in Helaman 15:13.
Summary: In accord with the reading of the original manuscript, restore the repeated their in Helaman 15:13 (“their great and their true shepherd”); there is some indirect evidence (based on the repetition of the articles a and the in similar constructions) that supports repeating the possessive pronoun for conjoined adjectives within the same noun phrase.