Well, first let’s be clear about what we are talking about. There are three types of editing:
Structural - which deal with the big items like plot, pacing and character motivation
Line editing - which looks at the prose, but doesn’t concern itself with the story.
Copy editing - which makes sure that all the rules of grammar (and the style guide) have been adhered to. This is the person checking for noun-verb agreement, making sure you didn’t use farther when you meant further, and knows the difference between fewer and less.
Many times the acquiring editor (the person who bought the book) will do #1.
When an “editor” is hired (by either the publisher or the author) they are usually doing #3, although sometimes an editor will do both #2 and #3 at once.
The number of possible mistakes in any given manuscript numbers in the millions. I kid you not. Even if the editor found 99.9% of the errors, a significant number would still slip through. It’s not the fault of the editor, it’s just a recognition of the enormity of the task. I’ve yet to see any book, even something like Harry Potter, that is error free. It’s just not possible.
Now the publisher will hire one editor and one proofreader, and between those two people and the author, they’ll get a pretty high-quality book. I’ve never heard of a traditional house hiring two editors because they feel 1 is “good enough.”
Now, I’ve been using 2 editors for years now, because I find the deficiencies of one is made up for by the strengths in the other. Plus, it’s helpful to have two pairs of eyes. Again, no one can see everything. That’s my approach…and I can have that luxury because my books earn well. For other authors, the budget might limit them to one…but again, I’d say that it’d be better to hire two editors for say $800, than one for $1,600.
Yes, even at the highest professional level, NO EDITOR will catch everything. As I said, the number of possible errors is just too great. Working on bestsellers and winning awards means they are at the top of their game. But even so, they won’t be able to catch it all.