Forms Authentication Timeout vs Session State Timeout

Hi,
I've read from here:  http://codeasp.net/blogs/vivek_iit/microsoft-net/848/forms-authentication-timeout-vs-session-state-timeout  that it's better to set Forms Authentication timeout value to be greater than the Session State Timeout value.
In this case, I need to inform the user that the session state is about to expire AND also to tell user the authentication ticket is about to expire.  Seems like a double-work (if I set different values for Forms Authentication and Session State). 
Is there a way to use a popup alert and countdown to alert user of the situation in each case, and have a button that the user can click to sort of "Keep me logged in"?
Appreciate any help.