Scholar Emanuel Tov notes that the Dead Sea Scrolls do not prove single or multiple authorship of Isaiah.
Emanuel Tov "Exegesis and Theology in the Transmission of Isaiah," in The Unperceived Continuity of Isaiah, ed. James H. Charlesworth (T&T Clark: London, 2019), 96
2. More Than One Author of Isaiah? No Evidence from Qumran
One of the most debated issues in the critical research of Isaiah is its possible multiple authorship. Critical scholarship posits the composite nature of the book named Isaiah as created by the juxtaposition of two or three literary compositions. Scholars usually make a distinction between the writings of Isaiah son of Amoz living in the eighth century BCE in chs. 1–39 and that of Deutero-Isaiah living in the second half of the sixth century in chs. 40–66. Often the chapters of the second part of Isaiah are subdivided into Deutero-Isaiah (chs. 40–55) and Trito-Isaiah (chs. 56–66). Presumably at one point these two or three books were combined and at a still earlier stage Isa. 36:1–39:8 had been copied partially from 2 Kgs 18:13–20:19 and added as an appendix to the words of Isaiah son of Amoz, just like 2 Kgs 24:18–25:30 was appended to Jeremiah 1–51 as ch. 52.
Not all scholars accept these critical assumptions, and when the Dead Sea scrolls were found, there were great expectations that an answer to this vexed dispute would be found. However, no evidence has been found in any of the scrolls either to prove or disprove the composite nature of Isaiah. These scrolls are simply too late in order to show any evidence. I might add that the copying of the large Isaiah scroll by two different scribes (Isa. 1–33 and 34–66) is irrelevant, since this division of work reflects a mere scribal convenience of dividing the copying of this long scroll into two exactly equal segments (cols. I–XXVII, XXVIII–LIV). In my view the two segments of the scroll were copied separately in such a way that when scribe A finished his work he had to leave three lines open at the end of Isaiah 33 in col. XXVII, while scribe B started at the beginning of a new column.