RCW 80.01.070

The commission shall have full power to make joint or concurrent investigations, hold joint or concurrent hearings, and issue joint or concurrent orders in conjunction or concurrence with any official, board, or commission of any state or of the United States, whether in the holding of such investigations or hearings or in the making of such orders the commission functions under agreements or compacts between states or under the concurrent power of states to regulate interstate commerce or as an agency of the federal government or otherwise. When necessary the commission may hold such joint hearing or investigation outside the state.