Upvote:1
Because you have to claim and recheck bags when transiting through the USA, you could add your second checked bag at that point. Delta has a baggage counter after you clear immigration and customs in Atlanta. Just tell them you are tired of carrying your "carry on" bag and want to check it through.
Likewise if you are planning on picking up more stuff in Atlanta, you can likely check the bag at the check in counters as well. As long as you are not exceeding your baggage allowance, you should be OK. It is not really any different than gate checking an oversized carry-on bag.