Splitting a window is useful for viewing two different sections of a document at the same time. You can edit the last line of a document while viewing the first line.
To enable split window, go to Window > Split Horizontally or Split Vertically. You can drag the split border to adjust the split. You can split again to have 3 horizontal/vertical panes or 2×2 panes.
You can stop editing in a split window by selecting Remove Split from the same menu or just drag the border to the very edge.
To show two different documents across a split window, go to Compare > Split View between … and …
If you want to have two different documents open in different windows, read Document Groups.