Share user session across multiple subdomains?

I would do something like this:
http://demianlabs.com/lab/post/multisite...ual-hosts/

But keep two index.php files, one in /app1/ and one in /app2/ and manually change application path. No need to add PHP logic like the tutorial.

These should be the same in both application/config/config.php
$config['cookie_domain']    = "domain.com";
$config['sess_cookie_name'] = 'ci_session';

Hi admin, I'm new on codeigniter, (but not php or programing ) that same cookie domain does not works for me my sites are on same domain but different sub domains, on same server, different mysql db's and totally different user table sutructure.
what is your advice?