The fair place to meet is Kansas City, MO — the city closest to the midpoint of Boston and San Diego. From the farther side that’s about 27 hr 59 min of driving.
Recommended midpoint
Kansas City, MO
From Boston
26 hr 10 min
1,248 mi to Kansas City
From San Diego
27 hr 59 min
1,334 mi to Kansas City
Boston has the shorter trip; the split is off by about 1 hr 49 min. The alternatives below can even it out. Boston and San Diego are about 2,579 miles apart.
Boston and San Diego are about 2,579 miles apart by road. Split the difference and you arrive near Kansas City, the city closest to the halfway point between them. That puts roughly 26 hr 10 min of driving on the Boston side and 27 hr 59 min on the San Diego side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Kansas City still makes a fair, central place for Boston and San Diego to converge, splitting the travel instead of asking one side to cross the country.
If Kansas City doesn't have what you're after, Oklahoma City and Omaha are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Boston and San Diego.
Kansas City, abbreviated KC or KCMO, is the largest city in the U.S. state of Missouri by both population and area. It is located on the Missouri River at its confluence with the Kansas River, within Jackson, Clay, Platte and Cass counties. Read more →
Boston is the capital and most populous city of the U.S. state of Massachusetts. It serves as a cultural and financial center of New England, a region of the Northeastern United States. Read more →
San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. 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.