Xcode open two editor windows with same file

Xcode 5.1 doesn't have the "Open in Separate Editor" menu option mentioned in the accepted answer. By digging through Xcode Preferences, I found hints (under Navigation) that led me to this solution:

Option + Shift + Click on the file in the project navigator. In the popup that appears, the leftmost choice is "Open in a new window".

To remember this, I found it handy to think of it as adding the Shift key to the Option + Click command that opens the file in the assistant editor.

To choose the "Open in a new window" item in the popup, you can either navigate to it via mouse clicks then double-click on it, or use the keyboard: Command + Left Arrow, then Return.

In XCode 4 you can have a split window via View -> Show Assistent Editor.

You can then select the file to display in the main editor by clicking on it, and the file in the assistent editor by holding down alt (or: option) + click on it.