It actually uses the same session file that is created on the server, effectively sharing the same session variables between two web browsers.
Session variables are not stored on the individual user's computer or the web browser - they are stored on the web server, defined in the PHP.INI file. The file that is created is a simple text file with serialised information, relating to the session variables you define when creating the $_SESSION array. The session key/ID is part of the filename of this text file.
To be honest, I cannot think of any good reason why you want to do this, only that it is possible. It certainly isn't "best practice" to share session keys/IDs, simply because the information stored is supposed to be secure.