As its name implies, the View > Split View command is used to split the screen horizontally and thus be able to simultaneously examine code in two separate parts of the same file but distant from one another or different files. When the user no longer wants the split view, there is apparently no menu command to remove it.
It would be good – and, I think, very intuitive – that when the user does View > Split View, the menu command changes to View > Unsplit View (or something similar), so the user can easily toggle split view on and off with a keyboard shortcut, rather than having to clicking the red “X” in the lower (or upper) view.