The 1858 Wright edition dropped the repeated determiner their here in 1 Nephi 22:12. This error was not followed by the subsequent RLDS 1874 edition, which derives (in part) from the 1858 edition. This omission of the repeated their was a typo since elsewhere that edition (as well as every other one) has retained this repetition:
For further discussion and examples, see conjunctive repetition in volume 3.
Summary: Maintain the repeated determiner their in 1 Nephi 22:12 (“their Savior and their Redeemer”); the language here derives from Isaiah 49:26, which is quoted twice elsewhere in the Book of Mormon and always with the repeated determiner.