I have official Android 9.0 ROM for HU PX5 GS rk3368. However, I think is more like general question.
In my system, split-screen feature is disable (or just some apps are missing).
I tried to set
enable_freeform_support 1 but that did not helped.
Note I did it by:
adb settings put global enable_freeform_support 1
That is because Developer Options are also disabled (I can not enter - probably app / Activity removed)
I'm pretty sure there is a way to fix that, but I don't have enough knowledge about Android to figure it out. I have access to root (over adb)
After executing command and rebooting I checked if it was not reset to previous value
settings get global enable_freefrom_support
It returns 1 so it is ok. However feature is still not working.
I also find out that following prop may disable spliting screen
But it is not present in my configuration.