Atlanta and Pittsburgh are about 522 miles apart by road. Split the difference and you arrive near Charlotte, the city closest to the halfway point between them. That puts roughly 4 hr 45 min of driving on the Atlanta side and 7 hr 37 min on the Pittsburgh side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Charlotte suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Charlotte doesn't have what you're after, Cincinnati and Columbus are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Atlanta and Pittsburgh.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.