How to relate 2 user profile tables to a single login table? I have an Employee table , and another table named Student. Now I have a third table named the Login. 
Employee has emp_id as PK , and other specific attributes for an employee.
Student has std_id as PK , and other specific attributes for a student.

The login table has user_id as PK , but the user_id should also be FK from both employee and student. Means that if there is a profile for an Employee or Student then there will be a record for them in the login table. But I am not able to link the user_id of the Login table to both Employee and Student at the same time in a one-to-one relationship.

Could you please help me how to that. I am using MySQL for this.
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).