[Elladan and Elrohir]

I think that in the mentioned scene, Boromir is trying to get a compromise with Aragorn. He's basically saying, "No, you're not going to be king, but I'll accept you as my equal, as my fellow Lord of Gondor." By now he's spent substantial time with 'Gorn, 'Gorn's saved his life at least once, and so he's willing to extend the right hand of fellowship, though not (yet) willing to bow the knee. That's how I see it.