How to make 2 different session in one codeigniter ?

[eluser]Mario "St.Peter" Valentino[/eluser]
i have two folder application in one CI installation
1 for backend
1 for frontend

i open 2 application in same browser
ex: localhost/catalog/ -> this for frontend
localhost/catalog/admin.php -> for backend

first i login to backend then i open frontend in other tab then i open backend again
i wanna ask why i must login again or why session in backend destroy ?

anybody can solve this problem ?