How to run two sessions in same domains using Servlet

[Farakh khan]

Hello,

I have two totally different modules e.g. user and employees.
When I login to employees the user module and if I login to user module by logging out the employees then employee module throwing the following error:

HTTP Status 500 – Internal Server Error
java.lang.NullPointerException

These two modules or on the same domain e.g.

coderanch.com/employeesLogin 
coderanch.com/userLogin

Why when one module logins then other throwing NPE?
How can I run two different sessions on same domain?

Best Regards