Multiple php Session ID problem

[EricKara]

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

<?php 
session_id('reg-gen'); 
session_start(); 
session_destroy(); 
session_id('assess'); 
session_set_cookie_params(0); 
session_start(); 
and the rest of the code ................ 

AND php include file 2 
<?php 
session_id('assess'); 
session_start(); 
session_destroy(); 
session_id('reg-gen'); 
session_set_cookie_params(0); 
session_start(); 
rest of the code..............................

This works fine when I switch from one id to other without leaving the browser.
When I close the browser and try to access a page in the same area of the web site as previously (with the same phpinlude file) I am not asked to login.
I tried adding:

session_start(); 
session_destroy();

at the very beginning of each code above, but this time as I go from one page to other (with the same include file), each time I am asked to log in again.
How can I destroy the previous session when leaving the browser, but only need to log in once on the first page instance of the same site area?