As it stands, I have to manually load the previous chapter in the left half before starting to write in the right half.
To address this part first, there is a shortcut to loading two items into each split very simply: click normally on the thing you want to work on, then hold down the Alt or Option (on Mac) key on your keyboard and click on the thing you want to reference, in the other split. So what you’re wanting is two clicks away, which is about as efficient as it is going to get.
That said, have a look at the options in the Navigate ▸ Binder Selection Affects ▸ submenu. There is nothing quite like what you’re looking for, but you might find it more useful to force the binder to only automatically load stuff into one side, leaving you to manage the other split with Alt+Click.
As for why there isn’t something like what you describe, the overall problem with the idea is that you’re mapping a very human interpretation of something (a chapter) into something that has no real mechanistic “understanding” of what a chapter is. There wouldn’t be a simple (or even really complicated) way for the software to know what “Chapter 9” actually means, in terms of how your work is structured in the binder. Bear in mind, this program isn’t for exclusively writing books, never mind books with chapters in them. It’s a general purpose writing tool. If that doesn’t make sense, this longer post goes into how varied the concept of a chapter might be.