Such an excellent and important question!!
A more literal rendering of Matt 26:63, 64 (BSB) gives:
Then the high priest said to Him, “I charge You under oath by the
living God: Tell us if You are the Christ, the Son of God.” “You [pl] have
said it yourself,” Jesus answered. “But I say to all of you [pl], from now
on you [pl] will see the Son of Man sitting at the right hand of Power and
coming on the clouds of heaven.”
There are several things to observe about Jesus reply:
- It was in response to the question from the Jewish High Priest as to whether Jesus was the Messiah, the Son of God. Jesus does not deny this.
- The "you" Jesus addresses is plural - Jesus is addressing the all the people for whom the High priest was speaking - all the Jews and (later) Christians who accept Jesus as Messiah.
- Jesus also adds to what the High Priest says by then claiming the Jesus is not only Messiah, the Son of God (see Ps 110, Dan 7:13) but ALSO, the king of Jews who inherits David's royal throne (Luke 1:32, 33, John 1:49).
- Jesus then ADDS another dimension - they would also see Jesus coming in the clouds of heaven, presumably at a later date.
The New Testament makes much of this and often refers to Jesus occupying the throne in heaven at His ascension: Matt 26:64, Mark 14:62, 16:19, Luke 22:69, Acts 2:33, 7:55-56 (standing), Rom 8:34, Eph 1:20, Col 3:1, Heb 1:3, 8:1, 10:12, 12:2, 1 Peter 3:22. See also Ps 110:1, Matt 22:44, Mark 12:36, Acts 2:34, Heb 1:13, Rev 5.
We also see the NT regularly discussing the second coming of Jesus in the clouds of heaven: Dan 7:13, Matt 24:30, 26:64, Mark 14:62, Rev 1:7.
Thus, Jesus' reply in Matt 26:64 simply says that very shortly after that conversation, "from now on" (only three days later in fact); Jesus would be seated at the right hand of God in heaven; AND, thereafter, people would also see Jesus in the clouds of heaven as per Rev 1:7.
Bengel offers these insights in commenting on Matt 26:64 -
From this time forward, etc.) From this time forward, it shall
come to pass that ye shall see and know, by visible proofs, that I am
HE who shall sit on the right hand of power, and come in the clouds of
heaven. A pregnant mode of expression (sermo complexus). Henceforward
YE SHALL SEE me sitting and COMING. The return to judgment is
combined with the sitting on the right hand: and after the Lord’s
Passion they believed (see John 8:28), that which hereafter they shall
see. They did not believe in the past; therefore Jesus (as He
frequently did) appeals to the future. In the glory of Jesus this is
the first thing, that He is the Son of God: that He will come to
judgment is the last. The former is the foundation of the latter; the
latter the most glorious proof of the former.