How to create two editor panes for the same file?

IntelliJ IDEA 15

3 approaches

1. Right click on the file tab > Split Vertically / Split Horizontally

   

2. Being in the file that you want to split, press Ctrl + Shift + A > type "Split Vertically" / "Split Horizontally" > Enter

   

3. Being in the file that you want to split, Window > Editor Tabs > Split Vertically / Split Horizontally

   

Alt-Shift-Enter

In the 'Open File' or 'Go To Class' dialog, or Project view, press Alt-Shift-Enter (instead of just Enter) to have the selected file open in the current pane even if it's already open in another pane.

Shift-Enter will open the file in a floating window.

Older versions of IntelliJ

In older versions of IntelliJ (maybe before IntelliJ 2016): this shortcut was just Shift-Enter.

Newer versions of IntelliJ (2019.2)

Shift-Enter