How to relate 2 user profile tables to a single login table?
A Login maybe a Student or an Employee.

It sounds like you trying to define a FK relationship from Login to both Student and Employee, this is not correct.  You should be defining a FK relationship from Student to Login and Employee to Login

Edit for clarification:

If the PK emp_id for Employee and PK std_id for Student are the FK field then there can only be 1 for each given Login (user_id value).