A similar situation occurs in 37:5-7.
Interestingly, in both places, the original scribe left an unusual blank line. Perhaps he was aware of his omissions? Perhaps his exemplar was damaged at these points, given the physical proximity of the corrections?
These corrections offer good examples for paleographic study of the different hands in the manuscript. When I first began reading Hebrew manuscripts, I was often bewildered by paleographers' claims to be able to distinguish multiple hands within manuscripts and their corrections. The differences often seemed so small and the letter forms so inconsistent, that I found identifying different hands very difficult. I must admit, I am still far from expert in paleography, but I can pick up on much more now than when I first began. Reading through the Great Isaiah Scroll in particular, I have become intimately familiar with the original hand of the manuscript by sheer repetition. With this familiarity in the background, many of the differences in the corrections practically jump off the page now. And I'm not just talking about the size and color of the scripts. I think this would be a good point to show some of the distinguishing characteristics of the hands to show how such work is done.
* - Original Hand
1 - First Correction (34:17 - 35:2)
2 - Second Correction (37:5-7)
We could line up many such examples to show the precise differences between the different handwritings. These are merely a few letters as examples, but we can draw a few conclusions from them. The first correction is clearly done by a later scribe. The letter forms are quite drastically different from the original hand and exhibit later influences. These differences are so many and so striking that it is impossible to miss them, after you have been reading the Isaiah scroll for a while.
On the other hand, the second correction is written in a hand much more similar to the original hand. From these initial investigations, I suspect that it is a different scribe, due to slightly different letter forms. Nevertheless, because the two hands are so similar in many ways, we would have to look much more carefully to confirm this.
While this is very basic, I hope it helps clarify how scholars really can tell the difference between the handwritings of different scribes. Sometimes it is easy to tell, and other times it is more difficult. But it is always essential to understanding the history of the manuscript and its text.