This post discusses another aspect of the "two-sets-of-plates" scenario, looking at what Nephi said about the future Book of Mormon.
On this blog we've discussed the idea that Moroni put the abridged plates in the stone box on Cumorah, as indicated by the Title Page (the last leaf of the plates) and other extrinsic and historical evidence.
Joseph and Oliver completed the translation of those plates in Harmony, PA. Then Joseph gave those abridged plates to a messenger who took them to Cumorah. From the repository in Cumorah, the messenger picked up the plates of Nephi and took them to Fayette where Joseph translated them as 1 Nephi-Words of Mormon 1-11.
(see chart here: https://www.lettervii.com/p/the-two-sets-of-plates-schematic.html)
To review, the Lord told Oliver that when he finished helping Joseph translate "this record" (meaning the abridged plates), there would be "other records" that he would assist to translate.
1 Behold, I say unto you, my son, that because you did not translate according to that which you desired of me, and did commence again to write for my servant, Joseph Smith, Jun., even so I would that ye should continue until you have finished this record, which I have entrusted unto him.
2 And then, behold, other records have I, that I will give unto you power that you may assist to translate.
(Doctrine and Covenants 9:1–2)
38 And now, verily I say unto you, that an account of those things that you have written, which have gone out of your hands, is engraven upon the plates of Nephi;39 Yea, and you remember it was said in those writings that a more particular account was given of these things upon the plates of Nephi.40 And now, because the account which is engraven upon the plates of Nephi is more particular concerning the things which, in my wisdom, I would bring to the knowledge of the people in this account—41 Therefore, you shall translate the engravings which are on the plates of Nephi, down even till you come to the reign of king Benjamin, or until you come to that which you have translated, which you have retained;42 And behold, you shall publish it as the record of Nephi; and thus I will confound those who have altered my words.(Doctrine and Covenants 10:38–42)
IOW, Joseph didn't have the plates of Nephi in Harmony. He wouldn't get them until the messenger brought them to Fayette.
Aside from the extrinsic and historical evidence that Joseph Smith translated two separate sets of plates, we have a comment by Nephi that corroborates the two sets of plates scenario.
Nephi wrote, "And now, I would prophesy somewhat more concerning the Jews and the Gentiles. For after the book of which I have spoken shall come forth, and be written unto the Gentiles, and sealed up again unto the Lord, there shall be many which shall believe the words which are written; and they shall carry them forth unto the remnant of our seed." (2 Nephi 30:3)
2 And also, that I may remember the promises which I have made unto thee, Nephi, and also unto thy father, that I would remember your seed; and that the words of your seed should proceed forth out of my mouth unto your seed; and my words shall hiss forth unto the ends of the earth, for a standard unto my people, which are of the house of Israel;(2 Nephi 29:2)