Did these judges seem amenable to allowing you to split the summer?
If you applied/interviewed with the second judge after accepting the job with the first judge, then be sure not to let anyone know. That's bad form.
It looks bad if you back out of an internship just because you got something more prestigious. There's no sure way to tell how it will affect your career prospects - that's a question you might only be able to answer in retrospect, and even then you might not be sure. However, I wouldn't want to risk it.
Are you going to school in DC? If so, see if the second judge will allow you to intern during the school year rather than summer.