“… many of the Gentiles shall say: A Bible! A Bible! We have got a Bible, and there cannot be any more Bible.” reads 2 Nephi 29:3. Why do we need two sets of scriptures? “But thus saith the Lord God: O fools, they shall have a Bible; and it shall proceed forth from the Jews, mine ancient covenant people … O ye Gentiles, have ye remembered the Jews, mine ancient covenant people? Nay; but ye have cursed them, and have hated them, and have not sought to recover them. But behold, I will return all these things upon your own heads; for I the Lord have not forgotten my people.” First, the Lord reminds the Gentiles that they don’t care about what they already have. “Know ye not that there are more nations than one? Know ye not that I, the Lord your God, have created all men, and that I remember those who are upon the isles of the sea; and that I rule in the heavens above and in the earth beneath; and I bring forth my word unto the children of men, yea, even upon all the nations of the earth?” There are more than one land and “Know ye not that the testimony of two nations is a witness unto you that I am God, that I remember one nation like unto another? Wherefore, I speak the same words unto one nation like unto another. And when the two nations shall run together the testimony of the two nations shall run together also.” So we need two sets so God can prove that he remembers everyone and to show that he wants everyone to be able to know.
https://www.churchofjesuschrist.org/study/scriptures/bofm/2-ne/29?lang=eng&verse=#p