Steve and Bucky are together in Brooklyn before the war. They're both omegas, and for whatever reason, neither of them have an alpha in their lives. (Alphas are really rare in this universe? Alphas tend to be really overbearing to their omega mates and neither Steve nor Bucky is willing to put up with that? Steve has to be really careful not to get pregnant because of his health issues? Are they just not attracted to any of the alphas they know, or to alphas in general?) So when one of the guys' heats comes around, they help each other out.
The dynamic can be really close friends with benefits helping each other out, or something more conventionally romantic. Bonus points for dorky sex banter.