What's the Single Sign On Solution?

For SSO,there are two meanings:
1. only use one creditential info to sign on different web app.
2. Even more, Only need to sign one time. 
My case is: on one IIS server, there are 2 web application. Each app has it's own login page for user authentication (but the user's data is in same back database). With IIS, when user different web app, it will assign app a different session. For My case, I have implemented the solution 1.

My question is: how to make minimum changes to allow user only need to assign one time for differen web application. It means:
if user login into Wep app A, when they go Web App B in same browser window, No need to login again. 
or if user login into Wep app B, when they go Web App A in same browser window, No need to login again. 
 
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