R – External editor in Visual Studio 2008

You need to put your directory structure in your project directory. And then click "Show All Files" icon in the top of Solution Explorer toolbox. After that, the added directory will be shown up. You will then need to select this directory, right click, and choose "Include in Project."

Here's how to do it...

1. Select the tab you want two copies of
2. Select menu Window → New Window from the menu.
3. Right click the new tab and select New Vertical Tab Group

If New Window is not listed in the *Window menu note that the command does exist, even as of Visual Studio 2017. Add it to the Window menu using menu Tools → Customize → Commands. At that point decide where to put the New Window command and select Add Command.

UPDATED on "30 July 2018"

In Visual Studio Code version 1.25.1 and later

Way 1

You can simple left click on your file in the side-panel (explorer) and press Ctrl + Enter.

Way 2

Simply right click on your file in the Visual Studio Code side-panel (explorer) and select the first option open to the side.