I have two different session IDs ('reg_gen' & 'assess') for the same user to log in to different parts of the web site (containing several pages) with different passwords.
I use the following code in the two php include file to switch between sessions(and website areas):
php include file 1
Code:
<?php 
session_id('reg-gen'); 
session_start(); 
session_destroy(); 
session_id('assess'); 
session_set_cookie_params(0);
...