If you are working on an editor, I recommend that you use C# and WPF (or WinForms for a bit more straight-forward integration). It is easier, faster to develop, the tools are much better, and integrating C++ and .NET is relatively easy (if all you have to do is read/write files + some sort of in-game viewer).
That's pretty much all I'll be doing. A way to display what I've done so far, and the objects I'm trying to get into the game itself. So it would just be using the engine I'm currently making. Which is written in C++. So after long as I can connect C#/WPF with it, then It'll work.
The win32 API does not provide a native interface for separators, and by separators I think you mean the bars between adjacent child windows which the user could drag to resize those windows. While these are not very difficult to create "by hand", a not-so-complex editor can be made without those.
I'm one of the few that actually don't hate the win32 GUI API. It's object-oriented, light-weight (compared to the alternatives), it's native, and it has a rich set of controls. On the other hand, it's a C API, it's not easy to get into for lack of comprehensive introductory documentation, MS did not include a designer with the free versions of VS, and it's not so popular these days. But it's a very powerful library once you get your head around how it works and how to write your own custom controls, and most importantly, it's native, so you wouldn't have to deal with all that interop bulls**t to have the editor work with your native graphics engine, and you wouldn't have to learn C#/VB if you don't already know them.
Anyway, whether you end up using it or something else is up to you, and I wouldn't blame you for turning to another library, but maybe what you're aiming to accomplish isn't really that difficult to make in win32. From your description, a thin wrapper library like win32++ sounds like a perfect fit for your needs. It has lots of samples to start from.
I may try that out and see how well it works. Even without separators it would be fine, I was just looking for a solution / way to set it up. Even if i just had two windows instead of one. (Sort of like a GIMP setup)