Chris,
I am not aware of any special valves that would allow two locomotives to both have or share control of the automatic brakes. The steam engines did not gang the main reserviors together as is done with MU diesels. When doubleheading the lead engine controlled the automatic air brakes and the second engine cut out its automatic brake valve from the train.
My question in winter operation in the cold of the Gunnison valley is whether they could pump up enough air to get legal air pressure at the rear of a 125 car train. I know that even now with multiple unit operation it is sometimes necessary to shorten a train to get sufficient air pressure at the rear on cold winter days.
John Bush