Your question tells me that you do not yet understand the
Flex application architecture.
Flex apps are not page-based, they are component-based.
Instead of calling a new page to change the user interface, you
simply navigate to a different component. State is retained on the
client, and "sessions" are not used.
You will want to have a main applicatio file, say "main.mxml"
In that file you will have Viewstack. The first child of the
ViewStack wil be your signin component, another will be the signout
component, and the rest will contain the rest of your application.
Signin will display first. Whe sigh in is successful, your
will set the viewstack to the desired application view. When the
user needs to signout, set the Viewstack to that view.
The signin data can be kept at the main application level
where it can be easily accessed from anywhere in the application.