Try below logic.(Two application are app1 & app2. User name is u1)
1. U1 login to app1(In DB a new row will insert for storing app1 session)
2. u1 login to app2
1. Check any open session is available in DB (You can use Session_Start event for this)
2. if available then restore DB session(app1 session) to app2 application.
3. if not available start new session and insert into db (Do the same logic in app1)
If you have any doubt pls ask.