According to DRGW Colorado Division Timetable, dated 10/01/65, the following applies:
Rule 17-N. Trains must not be doubleheaded on descending grade movements Cumbres to Alamosa, Cumbres to Chama, Chama to Gato and MP 443 (near Falfa) to Carbon Jct, except that in snow service trains may be doubleheaded when authorized.
I don't know when this was added, but probably after the derailment of the 483 and 494 near MP 445 on 9/26/58, resulting in the death of fireman, Paul Mayer. The 1955 DRGW Timetable (4/24/55) clearly says that on Subdivision 12 (Chama - Durango) that when second engine is used, it will be placed on the head end.
I bet Earl can tell us when the "doublheading downhill rule" first showed up in the DRGW Timetable Special Instructions.