how to merge two columns value into single column in sql select statement?

i have a query like this
SELECT     tblClientDocument_Base.Document_name, tblJobDocument_Base.Document_Name AS Document_name FROM         tblClientDocument_Base JOIN                       tblJobDocument_Base ON tblClientDocument_Base.Tenant_Id = tblJobDocument_Base.Tenant_Id
it returns
Document_name        Document_name
a                                   d
b                                   e

Go to the complete details ...