(1) Where there are more plaintiffs than one, any one or more of them may be authorized by any other of them to appear, plead or act for such other in any proceeding; and in like manner, where there are more defendants than one, any one or more of them may be authorized by any other of them to appear, plead or act for such other in any proceeding.

(2) The authority shall be in writing signed by the party giving it and shall be filed in Court.