Autofeather is active on takeoff and approach in the planes I've flown. If you have a dual engine failure in either scenario, you're going down if one or both engines are feathered. My guess only one A/F at a time to prevent spurious dual A/F.
It's really there to protect for a V1 cut and if you lose both engines at V1, you're not getting off the ground unless you go off a cliff.