This is the hypothesis that the Book of Isaiah was a composite work by unknown authors (now known as I Isaiah and II Isaiah), whose lives and experiences were separated by many decades. An hypothesis is any unproven theory, and there is still no solid evidence for the Deutero-Isaiah hypothesis apart from wishful thinking, and so it is wrong. Jesus Christ quoted from both the first half and last half of Isaiah, and said the same Isaiah wrote both of them:-
Joh 12:37 But though he had done so many miracles before them, yet they believed not on him:
Joh 12:38 That the saying of Isaiah the prophet might be fulfilled, which he spake, "Lord, who hath believed our report? and to whom hath the arm of the Lord been revealed?" (Quoted from Isa 53:1)
Joh 12:39 Therefore they could not believe, because that Isaiah said again,
Joh 12:40 "He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them. " (Quoted from Isa 6:10)
Joh 12:41 These things said Isaiah, when he saw his glory, and spake of him.
The relevant Old Testament passages Christ quoted are:-
Isa 53:1 Who has believed our report? And to whom is the arm of the Lord revealed?
Isa 6:10 Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.
(Note that the parallel quotes from the testaments do not agree because the Old Testament was written in Hebrew, and Christ was quoting from the Greek translation known today as the Septuagint. )
This is confirmed by reputable scholars. Lee Campbell (http://www.xenos.org/classes/Isaiah/isweek2.htm) writing on Isaiah says:-
Authorship of Isaiah In spite of the lack of concrete evidence that any part of Isaiah ever existed without any other part as far back as the 200's BC, the dogma of most scholarship today is that two or more individuals authored Isaiah. This perspective arose, most notably in the deistic climate of 18th century Europe. J. C. Doederlein, one of the earliest to argue for a second author, said explicitly that since Isaiah could not have forseen the fall of Jerusalem, the 70 year captivity, the return or Cyrus, Isaiah could not have written those chapters making such claims (e.g. chapters 40-66). Since this time, others have advanced arguments in support of dual or even multiple authorship