Have you tried to rebook it? You should be able to cancel both tickets for credit so if you can rebook as one ticket at the same price your problem is fixed. Have you tried calling Delta customer service? If you call at the right time you can get through quickly now and if you get a good agent they may be able to help you. If your agent says nothing can be done, call back. I have had some really excellent agents and a few useless ones. If you take handbaggage you will have no problem in ATL. Once you are through immigration you can access any terminal.