Whatever it means to be in the top tier of applicants for either Duke or Brown, I think you are in it. Your profile is exceptional. Still, most exceptional applicants are rejected by both schools.
If you truly love Emory and what it has to offer, then by all means, you should apply there ED2. Do you really prefer Emory over finding out if you get in to either Duke or Brown RD? I ask because I have a son who applied ED to one of these 2 schools, was rejected in the ED round, and ended up graduating from the other. He had an incredible experience which led to a very fortunate career opportunity that came about specifically because he attended this institution. Don’t ED2, and pass up on other potential opportunities, unless your heart is really in it.