You can split the editor window into two parts. By default you use the horizontal splitter marked with the arrow.
This will create a split screen :
With the option Vertical Splitter, you switch between horizontal en vertical splitter.
The splitter is located near the code explorer window.
This will result in a vertical split window.
When you chose the vertical splitter option again the window will be split horizontal again.
Notice that in order to show two different code windows you need to open the two windows and use Tile Vertically.