Hyrum given sealing power, made co-equal with Joseph. (1/19/1841)
And again, verily I say unto you, let my servant William be appointed, ordained, and anointed, as counselor unto my servant Joseph, in the room of my servant Hyrum, that my servant Hyrum may take the office of Priesthood and Patriarch, which was appointed unto him by his father, by blessing and also by right; That from henceforth he shall hold the keys of the patriarchal blessings upon the heads of all my people, That whoever he blesses shall be blessed, and whoever he curses shall be cursed; that whatsoever he shall bind on earth shall be bound in heaven; and whatsoever he shall loose on earth shall be loosed in heaven.
And from this time forth I appoint unto him that he may be a prophet, and a seer, and a revelator unto my church, as well as my servant Joseph; That he may act in concert also with my servant Joseph; and that he shall receive counsel from my servant Joseph, who shall show unto him the keys whereby he may ask and receive, and be crowned with the same blessing, and glory, and honor, and priesthood, and gifts of the priesthood, that once were put upon him that was my servant Oliver Cowdery; That my servant Hyrum may bear record of the things which I shall show unto him, that his name may be had in honorable remembrance from generation to generation, forever and ever. (D&C 124:91-96)
Never but one? (1831/1843)
"And verily I say unto you, that the conditions of this law are these: All covenants, contracts, bonds, obligations, oaths, vows, performances, connections, associations, or expectations, that are not made and entered into and sealed by the Holy Spirit of promise, of him who is anointed, both as well for time and for all eternity, and that too most holy, by revelation and commandment through the medium of mine anointed, whom I have appointed on the earth to hold this power (and I have appointed unto my servant Joseph to hold this power in the last days, and there is never but one on the earth at a time on whom this power and the keys of this priesthood are conferred), are of no efficacy, virtue, or force in and after the resurrection from the dead; for all contracts that are not made unto this end have an end when men are dead." (D&C 132:7)
Hyrum was Joseph's chosen successor
"Did Joseph Smith ordain any man to take his place? He did. Who was it? It was Hyrum..." (Brigham Young, Times & Seasons, October 15, 1844, Vol. 5, p. 683.)