Yes.If you have only one file open and then click "Window > Split Vertically", you get the same file on both sides. At least in IntelliJ IDEA 6.0.

I'm afraid it's not possible in the recent IDEA versions. Please vote for IDEA-34593 or watch this issue for progress to get a notification when the issue is resolved.

UPDATE: As the issue was fixed, it's possible since IDEA 10, see the reply from MattLeidholm.

It's possible in IntelliJ 10. First, use "Window -> Split Vertically" like Mr Rothgiesser suggests; this will open the same file in two panes in your single window. But then, to get it in a separate window, click and drag the tab from the newly-created pane off the original window into its own space on the desktop (that sounds more complicated than it is--just drag the tab off the window).

The file is now open in both windows, synchronized just as Mr Midwood suggests.

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

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

   

Press Shift-Ctrl-Enter if you use IdeaVim and want same file get opened in an active tab group.

IntelliJ IDEA 2021.2

Right click on the file tab > Split Right / Down