This is how to open the same file (for example, RecurrenceImpl.java) in a split editor window in Eclipse:
Right click your file name (i.e. the tab title)
Select New Editor
Now you have two files with the same name opened in the same editor window
Left-click (don't release) on a RecurrenceImpl.java's tab, drag the cursor to the lower edge of the active editor, wait a second so that the cursor changes into a black filled arrow, and release the mouse button.