On that you would feather both as if you could do only one the other would still be causing drag and trying to turn the engine…
On the Shackleton installation, the pitch control mechanism acted on the front prop. As the blades of the front prop moved, pinions mounted on the blade roots drove racks, which in turn moved a translation unit in a fore and aft direction. The translation unit was effectively a large bearing, one part of which rotated with the front prop, while the other rotated in the opposite direction with the rear prop. The fore and aft movement of the T.U. was transmitted by another lot of racks to pinions on the rear prop blades, so that they changed pitch (O.K. so far?)
The failure of a single rack could result in one of the six blades going fully fine. Trying to feather that prop resulted in five blades feathering with the sixth staying fully fine. Apparently this gave a slow rotation, some vibration, and some drag.
One one occasion we had a T.U. break up. There was some fire in the prop hub, where there was no extinguisher but that soon went out when all the bearing lubrication had burnt up. What was much worse was that when we tried to feather the prop, the front prop feathered but the rear went fully fine. Only about 300 R.P.M, no vibration, but an awful lot of drag. We were heavy, and were struggling with full power on the other three Griffons. Started the Vipers, but this was in the early days of the Viper installation when they could only run at full power or idle, and full power was limited to only a few minutes. (They later put in a continuous setting of around 95%)
Captain called for fuel jettison. Rather alarming, as the fuel jettison pipes were only a few feet from the Viper jet pipes, and Pilots Notes did not cover the subject. Got the weight down, and eventually landed safely.
Captain phoned Boscombe to ask why there was nothing in the notes about jettisoning fuel with the Vipers running. He was told that they had not tried it as it was considered too risky.