It appears that there is an internal ability to dock Split Panels. You might have seen this in the Folders Pane.
It would be great to expose this as a Public API and setting in Studio.
And to expand
Thank you for the idea, we will try to introduce public API for this in one of the next releases: https://youtrack.cuba-platform.com/issue/PL-10348
We have implemented this API in the upcoming version 6.9!