Right now, I dont know how to do it in C++, but it should be possible already, via changing the runtime, what means you have to recompile the whole engine ;(.
I would like to see the following features exposed via blueprints to change viewports: sizeX, sizeY, originX, originY.
I know this, because I found the structure thats used for the splitscreen data:
When we would have that interface, we would be able to do whats seen in the video under the link you posted.
The code to create custom splitscreen viewports can be found here:
‘Engine/Source/Runtime/Engine/Classes/Engine/GameViewportClient.h’
‘Engine/Source/Runtime/Engine/Private/GameViewportClient.cpp’
‘Engine/Source/Runtime/EngineSettings/Classes/GameMapsSettings.h’