Isaiah 10:12 (King James Bible) when the Lord hath performed his whole work upon mount Zion and on Jerusalem
In both of these passages, we have increased parallelism in the Book of Mormon text. In the first case, the Book of Mormon text repeats the preposition to. The Hebrew text has the prepositional prefix l- before both Jerusalem and her idols. In the second case, once more the Hebrew text has an identical prepositional prefix, this time b- before both mount Zion and Jerusalem. Thus in these instances the Book of Mormon text more faithfully represents the parallelism in the original Hebrew than does the King James translation.
It should be noted, however, that the Book of Mormon text does not consistently increase the parallelism. For instance, in the first example, the previous clause reads identically in the Book of Mormon and the King James Isaiah as “shall I not as I have done unto Samaria and her idols”. In the Hebrew, the prepositional prefix for unto is the same l- that is found in the following clause, but there it was translated in the King James Bible as to rather than unto (“to Jerusalem and her idols”). Moreover, the Hebrew prepositional l- is repeated before both Samaria and her idols; yet the Book of Mormon text does not repeat the unto, perhaps because the repetition of the longer unto is rather awkward (“unto Samaria and unto her idols”). Nor was there any attempt in this case to change the preposition unto to the simpler to—that is, “to Samaria and (to) her idols”.
Despite this one remaining case of nonparallelism, the two cases of increased parallelism here in 2 Nephi 20:11–12 seem intentional. For other examples of increased parallelism in the Isaiah quotations, see the discussion for each of the following:
In general, cases of increased parallelism appear to be intentional and will therefore be retained in the critical text.
Summary: Follow the more parallelistic Book of Mormon readings in 2 Nephi 20:11–12 (“to Jerusalem and to her idols” and “upon mount Zion and upon Jerusalem”).