The vision given to John is in regard to the Millennial kingdom, and the thrones which he saw were these thrones:
"That ye may eat and drink at my table in my kingdom, and sit on thrones judging the twelve tribes of Israel" (Lk.22:30).
the following verse is speaking of only "one" throne:
"And there shall be no more curse: but the throne of God and of the Lamb shall be in it; and his servants shall serve him" (Rev.22:3).
Here is what John saw and heard while he was at that throne, the throne of God and of the Lamb:
"And he that sat upon the throne said, Behold, I make all things new. And he said unto me, Write: for these words are true and faithful. And he said unto me, It is done. I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely. He that overcometh shall inherit all things; and I will be his God, and he shall be my son" (Rev.21:5-7).
This is God speaking and since there can only be ONE who is the beginning and the end and the Alpha and the Omega then the one speaking is also Jesus Christ:
"And, behold, I come quickly; and my reward is with me, to give every man according as his work shall be. I am Alpha and Omega, the beginning and the end, the first and the last" (Rev.22:12-13).
These are the words of the Lord Jesus because He is the one who says,
"I come quickly" (Rev.22:20).
We can also see that the Lord Jesus is called the "Almighty":
"Behold, he cometh with clouds; and every eye shall see him, and they also which pierced him: and all kindreds of the earth shall wail because of him. Even so, Amen. I am Alpha and Omega, the beginning and the ending, saith the Lord, which is, and which was, and which is to come, the Almighty" (Rev.1:7-8).