What is the Deutero-Isaiah hypothesis?

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 an unproven theory, and 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