Complex sentence: Mormons believe that the Urim and Thummim of Joseph Smith and the Book of Mormon were the functional equivalent of the Urim and Thummim mentioned in the Old Testament, but there is no indication that the Old Testament Urim and Thummin were used to translate documents.
Split into simpler sentences: Mormons believe that the Urim and Thummim of Joseph Smith and the Book of Mormon were the functional equivalent of the Urim and Thummim mentioned in the Old Testament. There is no indication whether or not the Old Testament Urim and Thummin were used to translate documents.