This is a fair question. Though I have maintained the traditional authorship of Isaiah by a prophet of the same name living in the 8th century BC, what difference does it make to Revelation if in fact the book was a compilation of two or more authors? In the first place, I point to the Pontifical Biblical Commission's 1908 statement on Isaiah that there is no good reason to doubt a single authorship (see here). But beyond this statement, how does asserting a second author to Isaiah in particular (or multiple authors to any biblical book in general) undermine faith? In the words of the commentator, where is the "necessary link" between multiple authorship and heterodoxy?
I personally think the link is not in the fact of multiple authors, but of the chronology one builds around those alleged authors. There is nothing inherently wrong with postulating multiple authors of the Scriptures. At the minimum, we already have 50+ human authors to the Bible, probably a ton more if you factor in scribal additions to the Old Testament throughout the centuries. What is the real difference whether we posit 50 authors or 75 authors? The Church acknowledges that God inspired these authors, and so who they are is not entirely of that much importance in and of itself (although I would suggest identity is much more critical in the New Testament). Was II Samuel the work of a single scribe or mutliple scribes over decades? These type of questions, in and of themselves, are not problematic.
They do become problematic in two cases, however (1) When the book in question is prophetic in nature, and when (2) positing additional authors causes us to shift the date of the composition of the book to beyond the events prophesied.
Say we take the Book of Daniel. Daniel traditionally was composed during the period of the Exile, sometime between 550-450 BC. However, the book prophesies many events that do not occur until the time of Alexander and then some things that occur in the time of Christ. Now, suppose we look at these prophecies from an anti-supernaturalist viewpoint by assuming that they could not be legitimate. If we take this as our axiom, then the only way we can explain away these prophecies (which would otherwise be miraculous) is by saying that the prophetic portions of the book must have been written in what is called ex eventu ("after the event") narration. This means there must have been a second author who added to the book of Daniel, in a sense writing prophecy backwards to make it look like Daniel had made accurate prophecies when in fact they were written by some other person after the events prophesied had already come to pass (see this article on Daniel's historicity).
Now we have come to a place where the assertion of a second author causes a real problem, for by saying that this second author came centuries later and added ex eventu prophetic portions to Daniel, we are in effect denying the supernatural prophecies of the book and thereby denying the supernatural nature of revelation; Daniel (or Isaiah, or whatever) clearly says such-and-such is a prophecy still to come, but by positing a second author, we offer a naturalistic explanation for the prophecy and rob these passages of their supernatural character.
Interestingly enough, the PBC condemns this thinking in the same response in which it deals with the question of Second-Isaiah. The following position is condemned:
Regarding Isaiah in particular one notices an abundance of prophecy. The Exile and return are foretold; the name of the king who would issue the edict of return is stated (Cyrus); future judgments on Egypt and the nations are described that later come to pass, not to mention all of the Messianic prophecies found throughout Isaiah. Now, if Isaiah prophesies the Exile to Babylon and the return, this is truly miraculous, given that these events did not occur until almost two hundred years after Isaiah. But if we say there was a Second Isaiah writing after the exile, then we can just say "ho hum" when the book makes these prophecies, for we have vacuumed out the supernatural, or in the words of the PBC, asserted that the alleged prophecies are simply "narrations put together after the event." And this is what Second Isaiah is all about; do a simple Wikipedia search on "Deutero-Isaiah" and you will find this explanation:
Passages of Isaiah 40-66 contain some events and details that did not occur in Isaiah's own lifetime, such as the rise of Babylon as the world power, destruction of Jerusalem, and the rise of the Persian king Cyrus the Great and his destruction of Babylonian Empire.On the other hand, the first section of Isaiah saw the destruction of Babylon at the hands of the Medes and the Elamites(13:1-20, 21:2) . This is generally explained by either considering Isaiah to have been given such information by divine means, or by considering the later sections of the book to be, not written by Isaiah, but written by those who lived later than Isaiah himself. Those that reject the supernatural revelation of God's foreknowledge to Isaiah hold to the second explanation and the mainstream scholarly understanding.
Therefore, yes, positing multiple authors can be very damaging to faith, if they involve prophetic books and chronologies.
Is there ever a licit recourse to multiple authorship? Sure. The PBC said, in its day, it saw no reason to posit more authors for Isaiah, and nor do I. That's not to say there couldn't have been, only that the PBC saw no necessity in arguing for them. But let's say that maybe Isaiah dictated his prophecies to a series of scribal pupils who compiled them over several decades. This is highly possible and would account for various stylistic variations. Let's say Isaiah wrote half of it and the latter portion was composed by pupils after his death who nevertheless heard his words, just like Aquinas' pupils finished the Summa for him. That is plausible, too. But if you are going to say that it was added to centuries later by persons who wrote in prophecies retroactively, then that is damaging to faith.
So, to answer the commentator, it is not simply sufficient to say one the one hand that we believe the author is God and that the Church gets the true context if, on the other hand, we take up critical exegetical positions that lead us to deny everything supernatural about the book.


stubbornly clung to and selfishness yet to be rooted out.