So, do you have any content, other than the one homepage, that is the same for both offices? I would think there will be a need for some such content - maybe a general "About The Firm" page, or other details that are going to be the same for both offices, and would also create duplicate issues beyond just the blog.
If so, is it too late for you to consider a single navigation, with a top level page for both Boston and Atlanta, and then subpages for each that can be optimized for your local terms?
Regarding duplicate content, you are correct that Google has stated that there isn't a penalty, you just will only get the version of a page that is most appropriate. So if someone is searching from Atlanta, they will most likely get the Atlanta version of the blog.
Will you have some blog posts that only are relevant (or are most relevant) for one location or the other? If so, duplicating a portion of pages is probably safer than 100% duplication.