The fair place to meet is Indianapolis, IN — the city closest to the midpoint of Cincinnati and Milwaukee. From the farther side that’s about 5 hr 6 min of driving.
Recommended midpoint
Indianapolis, IN
From Cincinnati
2 hr 29 min
99 mi to Indianapolis
From Milwaukee
5 hr 6 min
243 mi to Indianapolis
Cincinnati has the shorter trip; the split is off by about 2 hr 37 min. The alternatives below can even it out. Cincinnati and Milwaukee are about 324 miles apart.
Cincinnati and Milwaukee are about 324 miles apart by road. Split the difference and you arrive near Indianapolis, the city closest to the halfway point between them. That puts roughly 2 hr 29 min of driving on the Cincinnati side and 5 hr 6 min on the Milwaukee side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Indianapolis suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Indianapolis doesn't have what you're after, Chicago and Detroit are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Cincinnati and Milwaukee.
Indianapolis, colloquially known as Indy, is the capital and most populous city of the U.S. state of Indiana and the county seat of Marion County. Indianapolis is situated in the state's central till plain region along the west fork of the White River. Read more →
Cincinnati is the most populous city in Hamilton County, Ohio, United States, and its county seat. Read more →
Milwaukee is the most populous city in the U.S. state of Wisconsin. It is located on the western shore of Lake Michigan at the confluence of the Milwaukee, Menomonee, and Kinnickinnic Rivers. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.