Well, first, once the work is done it's not clear how you would even do that. And second just about everywhere I've worked has the front end and back end in different repos, so it would be 2 different PRs anyway.

I'm really just trying to understand what you're saying, where there's an existing PR out there how someone would break that existing PR up if they were asked to. I'm fairly senior, and I have no idea what that would look like--how I would physically do it. That's why I usually do submit PRs for individual pieces of functionality where appropriate (but this can cause issues in JIRA bc people want to mark a story "in test" when they see a PR deployed to test with the ticket number on it).

I just can't fathom asking someone to break up a PR they've already done, because if they asked me how I wouldn't have an answer. Now, I can tell someone "I don't care if it's done or not, submit it right now because your branch is getting too far out of synch." And I have.

It would be useful to be able to ask people to break up their PRs if there's actually a way to do it after the fact, but so far it seems like when you said that it was just a figure of speech and not something people on your team really do. Which is confusing.