I assume you are referring to the turnpike which connects
the 2 cities. Newville, PA is probably the closest to the midpoint.
There is a large State Police barracks & Penndot facility there, which
may be a reason for the central location also.
If you take interstate all the way, Washington DC would be a route. However, to enjoy a far more scenic route, which only adds approximately 40 more miles to the journey, travelling via Harrisonburg Virginia would be an alternate mid-point and permit you to travel through the mountain ranges.
Roanoke Red Sulphur Springs, VA is the halfway point.
There is 464 miles between Jacksonville, Florida and Chattanooga, Tennessee. The city that is halfway between these two cities would be Macon, Georgia.
Dunn, NC
Florence, SC
Walterboro, SC
Blumpkin City, FL
Pensacola, FL
Ocala, Florida is the halfway point between Tampa and Jacksonville. Ocala is the county seat of Marion County, and the 45th most-populate Florida city.
U.S. 301, Jacksonville, FL 32234
lincoln, nebraska
Hammond, Indiana
Oklahoma City is fairly close to halfway between Flint, Michigan and Tucson, Arizona.