There's an experimental flag '#more-split-screen' which shows that option. Not working yet but it seems the work in progress
2025-09-17_142541.png
I noticed another one flag #side-by-side , not clear what for because I don't see difference when enabling.
2025-09-17_142827.png
*In my opinion two more 'must have' options are 'swap' left/right screen and open link from left screen in the right one. And it was suggested/requested multiple times since the time the feature was introduced