Actors can play scenes opposite themselves (you film both coverages with a body double, so the camera is drawn to the face of the primary actor, and the eye accepts that the other one is the same person).
It is grueling work, though, because of course you need to be two characters in two costumes and differentiate them from each other in some way, as well as playing both sides for all of the shots. Some actors do it well. Some do it...less well.
I don't know if Benedict Cumberbatch has ever done that before; I don't know his work, so wouldn't know where to look for any examples. I would be just as happy to cast someone else as Elros, if we can, when we get there. But it's possible we'll have the boys be teenagers when they're together, so having the same actor play Elros and Elrond in very different scenes might not even matter, in the end. There's no indication that Elrond ever visited Numenor (though we may choose to have him and Elros meet up at some point after it's founding for some reason).
You may recall this scene from
Return of the King, where the Witch-king confers with Gothmog:
Naturally, this scene is
Lawrence Makoare interacting with
Lawrence Makoare, as he played both characters. But the average viewer doesn't notice or think that, given the types of costumes you're looking at there.
There are plenty of other examples of actors playing two versions of the same character, or an alter ego. No one is surprised that the same guy who plays Clark Kent is also Superman, or that Peter Parker is Spiderman, or that Bruce Wayne's actor is also Batman's actor. Granted, most of these involve separate scenes, but there are occasional blurred lines, where we have to believe that it's Bruce Wayne, but another character he's interacting with has to believe that it's Batman. Or something.
Edward Norton did an entire film where he played his own twin, and those twins were the main characters. Granted, I've never actually watched
Leaves of Grass, so maybe it's terrible, but it certainly has been done.
Clip (language warning, drug use)
They have different hair styles, clothing, mannerisms, personalities, and accents. It's easy to believe it's two different people.
And of course you knew I had to have some
Supernatural examples
Misha Collins talks to himself in a dark empty echoing room (The Empty)
Identical costumes, but a strong effort to differentiate characters with weird accents and mannerisms. Misha later joked that his accent in this scene is typical of people trapped on oil rigs in the Atlantic.
Jensen Ackles is Dean...and a subconscious dream version of Dean who is dressed and sounds exactly the same. You can always tell who is talking in this scene, which is just downright bizarre when you think about it. I'm not sure I could tell you how the acting is different, but it clearly is. It's little things: real!Dean does a lot more winking and has easy movements. Dream!Dean doesn't have that casual humor and moves quite deliberately. So his face is smoother and more fixed. But it's subtle and not nearly as easy to point out as the other examples (though of course there are all those subtle acting choices there too). It helps that the conflict between these two characters is established immediately in this scene; they can look and sound identical and yet clearly be different from each other.
So, long story short, if we have to have someone play their own twin, we can do it, but of course if someone can think of a look-alike to cast, we can do that too. If we wind up recasting teen Elrond and Elros, we can get twins for them, perhaps.