Sounds like you're breaking it up 10 hours the first day and 8 hours the second day, with you stopping in Atlanta around 4 pm the first day.
I always say, get just south of Atlanta before stopping, so you won't have to deal with Atlanta traffic the next day. However, it looks like you're leaving early enough the second day, so that morning rush hour won't be a factor. And you'll get into Atlanta just at the beginning of evening rush hour.
Sounds like you have two options.
1) Leave at 6 AM on day one and stop at 4 PM just north of Atlanta (or downtown Atlanta if you want to risk the traffic). However, the second day, I'd leave no later than 6 AM.
2) Leave no later than 5:30 on day one and stop at 4 PM just south of Atlanta. That way, you can leave about 6:30 the next day.
Have you taken breaks into consideration? From Northern KY, it takes us about seven hours to get to Atlanta, including breaks. And I know you're more than three hours north of us.