session id has overwrite when i refresh sub-domain site

[eluser]guzzilar[/eluser]
[quote author="InsiteFX" date="1342089504"]If the database session is store in the same table for both domains then thats your problem.

Each domain should have it's sess_cookie_name and sess_table_name different and each should have its own session table with a different name if running on the same database.
[/quote]
Thank's for your suggestion

but, when i checked session in both domain i don't store it into database , i use $this->session->userdata('session_id'); for check session id (check by refresh page in both domain toggle) and result is

First refresh: www.site.com >> e9687814977f4fe4286afa863337677d
Second refresh: sub.site.com >> 3bbed7255c4bc02880a0965133d60108
Third refresh: www.site.com >> 9a40d5e6923b6cf9921aa3598e8e8940

and if i use different db name for store session id , how i check session login?
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Thank for reply me hahaha