I was recently asked
If I hire you to edit my novel, what are your terms on copyright and editorial acknowledgment (say if there are 2 or more editors)?
My answer was simple: there aren’t any.
When you engage an editor, you’re buying “work for hire”, whether that’s specifically stipulated or not. You pay me to edit your project, and I do. I don’t have any claim on the edits themselves because I did them on your behalf, for money, as a job. I don’t own any copyright on your edited work — It’s still your writing, and you own it, fully.
You also don’t need to credit me in any way, unless we agree to that as part of the project. If we did, you’d know, and the agreement clearly laying out your responsibilities to that end would be in writing.
The only time you need to worry about copyright or an editor’s byline is when we’re talking about an acquisitions editor who purchases that story or novel on behalf of a publisher. The editor’s name might appear in the masthead of the magazine, or if it’s an anthology, the whole book would be “edited by”. If it’s a novel, the editor’s name doesn’t appear on the front of the book, and usually won’t appear inside it either, unless the publisher decides to include it in the frontismatter for some reason. Often, there are multiple editors, copyeditors, and/or proofreaders on a novel, and as an author, you might not even know who they all were.
A reader usually only knows who edited or acquired a novel if the author thanks them personally, or there’s a notice in a trade magazine that reads something like, “Bob McBobby, editor at BOB Publications, has purchased The Life of Bob.”
The fact that you hired a professional editor to help you tighten up your work before another editor purchased it doesn’t even need to come up when you sell it, unless they ask and you want to tell them. Until then, someone like me is equivalent to the mechanic who puts new brakes on your car when the others were squealing:
You need us to make everything go the way you want without anyone getting hurt, but if I do my job correctly, you’re the only one who needs to know you hired me. At least, until you recommend me to all of your friends.