((a)) The Chief Justice of the United States may, in the public interest, designate and assign temporarily any circuit judge to act as circuit judge in another circuit upon request by the chief judge or circuit justice of such circuit.
((b)) The chief judge of a circuit or the circuit justice may, in the public interest, designate and assign temporarily any circuit judge within the circuit, including a judge designated and assigned to temporary duty therein, to hold a district court in any district within the circuit.