By default, when you try to open a second instance (new window) of a running desktop app from the Start
screen, it will only switch to the already running desktop app instead of opening a new instance of the desktop app.
To open another instance of the same desktop app from the Start or Apps screen, you would normally need to either press and hold the Shift
key and click/tap on the desktop app OR
right click or press and hold on the desktop app and click/tap on Open new window
This tutorial will show you how to enable or disable to always have desktop apps open a new instance from the Start and Apps screens for only your user account in Windows 8