Hope this makes sense.

I bought two separate round trip tickets to get from ORD to Dothan, AL.

First set of round trip is ORD to ATL on UAL and return to ORD on DL. Second set of round trip tix on the same dates are ATL to Dothan, AL, and back to ATL. Both on DL.

The flight down shows I arrive on UAL in Terminal N. My DL flight to Dothan leaves a few hours later in Terminal S. Very confused about ATL, so do I leave the secured side from Terminal N to S? If I don't have to leave the secured side, am I ok to go from an UAL flight to a DL without checking in with TSA? Do I need to check in at the DL ticketing or gate?

Please help. In the past, I've always done connections and bookings with one airline to make it easiest, but the UAL flight in this mix saved me over $100.

Thank in advance.

It's the same terminal. The N and S refer to the location of the check-in counters. Once you clear security, everything in the domestic terminal is connected.