how to control session expiration for staff login and user login

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C#3.0

C#

ASP.NET

Hi Friends,
I my project i have to control the session using Response.AddHeader("Refresh", Convert.ToString((Session.Timeout * 60) + 5));
but the problem is :
1)Login as Staff through staff login page
2)Goto-->the page called Showlist.aspx and check the data available
3)Next Goto --> Simulation -->It simulate user menu
Now the session for staff already available and one session called session user created.
Then i trying to view the Showlist.aspx from user menu it can able to show what i expected.
but after i come to my staff page at that time i am clearing my session user variable to null
and try to open Showlist.aspx again in user page at that user session expired and it go to
userlogin page .
after that i come to staff page i am trying to open the staff session also expired .

Please help me to control asap.