Even Isaiah must have become a little impatient once he knew the Assyrians would pounce down upon the northern ten tribes and their ally, the king of Assyria. In any event, it looks as though this chapter was to assure Isaiah that the promised attack by the Assyrians would eventually come to pass.
The Lord wanted Isaiah to write a prophecy on a great roll -- not a slate -- but in ink on a great roll, predicting the coming war. And there had to be two credible witnesses to testify that the prophecy was recorded far in advance of the birth of Isaiah's son whose prophetic name would mean "Speed to the spoil, hasten to the prey" -- this is just another way of say "war is on the way."