Getting data from 2 tables by JOIN, when @Foreign Keys exist to the same Table/Object
You must use the User table twice:

SELECT N.text, S.name, S.email, R.name, R.email
FROM   Note as N 
   LEFT JOIN User as S on N.sender_id = S.id where S.staff is True       
   LEFT JOIN User as R on N.sender_id = R.id where R.staff is True