Folie à Deux, by cloud_wolfbane

The look Bucky gives him isn’t fear, it isn’t blame -even though it’s Steve’s fault- it’s resigned. He tries to reach out again, fingers grasping, but the last of the metal gives out and he tumbles off into the abyss. 

Steve doesn’t hesitate. He launches himself after him. The wind deafens him, beats against his face and blinds him, but he streamlines his arms and legs, propelling himself after his friend.

Explicit for violence and torture. There are two Winter Soldiers. This is intense and painful, partly from Steve/Soldaty’s POV, partly from Natasha’s.