The session will keep the information about each employee independently. i suggest you create an Application variable, whenever an employee logs in i.e. in session.start add his username to the Application variable(sort of list)(also, here you might have to dip in DB to check the user validity). once multiple employees are online the application variable will contain all their names.
now when a customer logs in, use this application variable and show the logged in employees names to him.
Let me know if my understanding is not correct. i will refine my answer.
EDIT - The Application valriable I am talking about is the state management technique which will allow you to save some values which will be accessed by all the users. check out msdn documentation for details. and see this article to get a cursory idea -
A Beginner's Tutorial on ASP.NET State Management[
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