This isn't possible (supported) for various reasons.
I miss this feature too. Really sad not being possible, but I understand the difficulty as the notebook has been chosen instead of the classic MDI model.
We use a notebook for various reasons. One of them is that in all non-windows platforms the MDI style is emulated with a notebook. So what do you think? Have MDI for windows and notebooks for other platforms? I don't think so. We strive for C::B to behave identically under all supported platforms.
Why are single-document-notebook and MDI the only two options?
As per my original comment, I anticipated that splitting the view would allow me to view two documents at once. But I only got that expectation after doing a split - I expected to be able to change which file one half of the split (at least) was showing.
What is the complexity in making it so that if you split a window - lets say horizontally - the bottom part of the split stays viewing the file it is viewing, but the top half is then controlled by the file tab.
Likewise for a vertical split the panel corresponding to the handedness of the locale (so left hand except for right-to-left handed languages) would be controlled by the tabs.