Indeed, doubleheading steam engines downhill was nixed by the Grande in the late 50's. Factor in the following. Concentrated weight of two locomotives on the point, the in-train forces created by long retainer trains, the gross weight of the train against the head end as it negotiates the sharp and relatively unballasted curves of an aging railroad grade.
We continued this procedure when we ran doubleheaders from 1981 forward. I do remember the recoupling of engines just west of the high bridge, but not how long that practice was continued.
By the way, 481's first trip was August 5th 1981. The "shakedown" run was combined with a celebration of the arrival of Durango's first train. The 481 backed from town to the Iron Horse Inn where a small compliment of coaches were staged on the main. A group of invitees in victorian attire boarded and rode into the depot. Supt. Jim Mayer was running and brought the train around the loop (ala Rio Grande days) so the train would appear heading in to the depot from the east for the celebration. Jim and I were remembering that day last night as I rode a yard goat around the yards here in Denver. Just found a few of my slides of this day recently!