So I got to thinking about different ways Hydra could have experimented with Bucky's head over the years and I got to thinking about personality imprints and it inspired this shameful trash idea.

Each time he's activated for duty the Winter Soldier is imprinted with a loyal Hydra soldier's personality and brain patterns to prevent 'incidents'. Given that he's one of their best men, Rumlow is often chosen as the imprint. It's not a perfect copy, but it's enough that Rumlow has a hell of a time working alongside his mini-me.

Cut to Steve facing Rumlow and Rumlow!Bucky in combat. Thanks to how in sync they are, they tag team him and manage to take him down... and then once he's down, they tag team him in an entirely different way.

+ Rumlow being narcissistic enough to really, really get off on sharing the honor of screwing Captain America with 'himself'
+ Rumlow and Bucky having a running dialogue as it goes on, and Rumlow's words coming out of Bucky's mouth utterly destroying Steve. (...and possibly turning him on?)
++++++ Rumlow and Bucky indulging in lots of stuff that's right out of Rumlow's personal porn collection; dirty talk, moneyshots, object insertion?