Unless there's a reason why they're separated (Natasha could be on a job on the other side of town or something), Natasha will totally do the whole second-in-command/duckling thing and just follow Steve around. (Sorry!) If they're separated, she will move mountains to make it to his side.

It would be nice to maybe have a scene where they can show off their really coordinated fighting styles to Bucky. Just sayin'.

Also, Nat will likely get one of those gross ear things. So she'll end up fighting whoever is close to her until they can empathy bond that.