http://www.intellij.net/tracker/idea/viewSCR?publicId=18990
Right now, there is no published notification of when an editor window is split, like openFile. This makes it hard to write plugins where we need to watch for focus change between the 2 panes (we cannot use selectionChanged callback in this case, must listen for focus). All one can do is, whenever an editor is created, recheck all the open TextEditors to see if they added any editors. (One could use the undocumented property change which is fired when a pane is split, and the undocumented userData item that shows which FileEditor an Editor is associated with.)