"God hath not revealed anything to Joseph, but what He will make known unto the Twelve, and even the least Saint may know all things as fast as he is able to bear them, for the day must come when no man need say to his neighbor, Know ye the Lord; for all shall know Him (who remain) from the least to the greatest. How is this to be done? It is to be done by this sealing power, and the other Comforter spoken of, which will be manifest by revelation." (Joseph Smith, HC v 3, ch 25, p 380)
"The doctrine or sealing power of Elijah is as follows:—If you have power to seal on earth and in heaven, then we should be wise. The first thing you do, go and seal on earth your sons and daughters unto yourself, and yourself unto your fathers in eternal glory, and go ahead, and not go back, but use a little wisdom, and seal all you can, and when you get to heaven tell your Father that what you seal on earth should be sealed in heaven, according to his promise. I will walk through the gate of heaven and claim what I seal, and those that follow me and my counsel." (Joseph Smith, TPJS Section 6, p. 340)
Notice Joseph refers to others holding the sealing power, and gives himself as an example of one who holds it and seals.
"Joseph understood when the authority is given to any couple who's calling and election is sure, they were expected to use it to secure for themselves their kingdom." (Denver Snuffer, Beloved Enos, ch. 11, p. 147)
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.)
"This is the election that we believe in, viz., such as we find in the Prophets and Apostles, and the word of the Lord Himself, and as we have not room to give all the quotations in full, in relation to election in this epistle, we would invite the Saints to examine the Scriptures, in connection with these quoted; and whenever they find election, or any other principle or blessing, given or applied to the house of Israel, let those principles continue with the house of Israel, and not apply that to Esau which belongs to Jacob; or to the churches of modern times which belong to the ancient covenant people; and always ascertain how the Lord, the Apostles and Prophets have applied their words, and ever continue the same application, and knowledge and wisdom will be added unto you; and in the words of the beloved Peter and Paul, we would exhort you to 'work out your own salvation with fear and trembling, for it is God which worketh in you both to will and to do of His good pleasure' (Phil. 2:12, 13); 'Giving all diligence to make your calling and election sure' (2 Peter 1:10); for this is that sealing power spoken of in Ephesians (1:13, 14)—'in whom ye also trusted, after that ye heard the word of truth; the gospel of your salvation, in whom also, after that ye believed ye were sealed with that Holy Spirit of promise, which is the earnest of our inheritance, until the redemption of the purchased possession, until the praise of His glory' (2 Peter 1:11); 'For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Savior Jesus Christ.' Amen." (Joseph Smith, HC 4:14)
"But how are they to become saviors on Mount Zion? By building their temples, erecting their baptismal fonts, and going forth and receiving all the ordinances, baptisms, confirmations, washings, anointings, ordinations and sealing powers upon their heads, in behalf of all their progenitors who are dead, and redeem them that they may come forth in the first resurrection and be exalted to thrones of glory with them." (Joseph Smith, HC 6:8)
Joseph ties the sealing power to having your calling and election made sure
"Peter exhorts us to make our calling and election sure. This is the sealing power spoken of by Paul in other places." (Joseph Smith, TPJS Section 3 p. 149)
"The servants of God are sealed in their foreheads, which signifies sealing the blessing upon their heads, meaning the everlasting covenant, thereby making their calling and election sure. When a seal is put upon the father and mother, it secures their posterity, so that they cannot be lost, but will be saved by virtue of the covenant of their father and mother." (Joseph Smith, Teachings, 321)