This was also the view of my own church here in South Africa on Isaiah
but the modern clincher was when the Dead Sea scrolls were discovered in
1957 they found the complete version of the prophet Isaiah’s book all
in the same handwriting by the same scribe and dated around 630 BC
Basically the dual authorship was propagated by people who disavowed the
prophetic content in the Bible and also thus not believing that GOD
knows the beginning from the end.
But the reference to Christ twice referring to Isias in the Gospels is just another way that the Bible declares itself.
All glory to our Coming KING of KINGS JESUS CHRIST
His Word shall ever stand