I built a plugin to support this feature as i also needed to build an “Editor-like” type of application.
You can read a more detailed answer here:
Split viewport into areas of different contents
But in short this is the plugin: Enhanced UI
It does NOT use Scene Capture, or Split Screen, or Materials…
The system simply takes the 3D viewport and places it in a UMG Widget…
Fab link : https://www.fab.com/listings/b8777f94-8b76-4af9-a110-3c6329b31d06