D&C 88

To a select group of High Priests, the Lord said -

"...the alms of your prayers have come up into the ears of the Lord of Sabaoth, and are recorded in the book of the names of the sanctified, even them of the celestial world.
3 Wherefore, I now send upon you another Comforter, even upon you my friends, that it may abide in your hearts, even the Holy Spirit of promise; which other Comforter is the same that I promised unto my disciples, as is recorded in the testimony of John.
4 This Comforter is the promise which I give unto you of eternal life, even the glory of the celestial kingdom..." (Doctrine and Covenants Section 88:2 - 4)

There are those that have assumed that this other comforter is our Savior, Jesus Christ, but these verses make it clear that this other comforter -

"...may abide in your hearts, even the Holy Spirit of promise..."

This other comforter then is the Holy Ghost with power to seal one to eternal life. The Lord said much the same thing in the 14th Chapter of John -

"...I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever;
17 Even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you." (New Testament John 14:16 - 17)

And of -

"...the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you." (New Testament John 14:26)

But when the Holy Ghost or the Holy Spirit of Promise that provides a promise of eternal life, the Lord said -

"He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him." (New Testament John 14:21)

In my mind then the Savior manifesting himself to him would be a "Second Comforter" or an additional comforter to that of the Holy Spirit of Promise. I believe this is what the Prophet Joseph Smith meant as he taught -

"...there are two Comforters spoken of. One is the Holy Ghost, the same as given on the day of Pentecost, and that all Saints receive after faith, repentance, and baptism. This first Comforter or Holy Ghost has no other effect than pure intelligence, . . . expanding the mind, enlightening the understanding, and storing the intellect with present knowledge. . . .

The other Comforter spoken of is a subject of great interest, and perhaps understood by few of this generation. After a person has faith in Christ, repents of his sins, and is baptized for the remission of his sins and receives the Holy Ghost, (by the laying on of hands), which is the first Comforter, then let him continue to humble himself before God, hungering and thirsting after righteousness, and living by every word of God, and the Lord will soon say unto him, Son, thou shalt be exalted. When the Lord has thoroughly proved him, and finds that the man is determined to serve Him at all hazards, then the man will find his calling and his election made sure, then it will be his privilege to receive the other Comforter, which the Lord hath promised the Saints, as is recorded in the testimony of St. John, in the 14th chapter, from the 12th to the 27th verses. . . .

"Now what is this other Comforter? It is no more nor less than the Lord Jesus Christ Himself; and this is the sum and substance of the whole matter; that when any man obtains this last Comforter, he will have the personage of Jesus Christ to attend him, or appear unto him from time to time, and even He will manifest the Father unto him, and they will take up their abode with him, and the visions of the heavens will be opened unto him, and the Lord will teach him face to face, and he may have a perfect knowledge of the mysteries of the Kingdom of God; and this is the state and place the ancient Saints arrived at when they had such glorious visions—Isaiah, Ezekiel, John upon the Isle of Patmos, St. Paul in the three heavens, and all the Saints who held communion with the general assembly and Church of the Firstborn." (Teachings, pp. 149–51.)