2 relationships between 2 entities in ER diagram

Your relationship for employs and manages are going to be two linking tables. e.g. Consider the tables and fields as below:

Department
   DepartmentID
   DepartmentName

Employee
   EmployeeID
   EmployeeName

SpecialEmployee
   EmployeeID
   Attr1
   Attr2

Now you can go on and link Employee and Department as such:

DepartmentEmploys
     DepartmentEmploysID
     DepartmentID
     EmployeeID


ManagesDepartment
     ManagesDepartmentID    
     EmployeeID
     DepartmentID

Now your relationships do not overlap or interfere. What seems to be drawn in your diagram is correct. There can be more than one relationships between two entities.

There is no reason you can't have multiple relationships between the same two entities, your understanding is wrong in this respect.

You have to be certain, of course, that the relationships capture different information and that you haven't, say, mistaken the two aspects of a single relationship as two different relationships. Your example seems to be a good candidate for having two relationships between the two entities.

Better approach is to add a bit field in Employee table IsManager, so if he is a manager, that means that he is a manager on department he works.

Hi mate watch this diagram, hope it helps