The fair place to meet is El Paso, TX — the city closest to the midpoint of Phoenix and San Antonio. From the farther side that’s about 10 hr 31 min of driving.
Recommended midpoint
El Paso, TX
From Phoenix
7 hr 15 min
345 mi to El Paso
From San Antonio
10 hr 31 min
502 mi to El Paso
Phoenix has the shorter trip; the split is off by about 3 hr 16 min. The alternatives below can even it out. Phoenix and San Antonio are about 847 miles apart.
Phoenix and San Antonio are about 847 miles apart by road. Split the difference and you arrive near El Paso, the city closest to the halfway point between them. That puts roughly 7 hr 15 min of driving on the Phoenix side and 10 hr 31 min on the San Antonio 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. El Paso still makes a fair, central place for Phoenix and San Antonio to converge, splitting the travel instead of asking one side to cross the country.
If El Paso doesn't have what you're after, Albuquerque and Tucson are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Phoenix and San Antonio.
El Paso is a city in and the county seat of El Paso County, Texas, United States. It is the 22nd-most populous city in the U.S., sixth-most populous city in Texas, and the most populous city in West Texas with a population of 678,815 at the 2020 census. Read more →
Phoenix is the capital and most populous city of the U.S. state of Arizona. With over 1.6 million residents at the 2020 census, Phoenix is the fifth-most populous city in the United States and the most populous state capital. Read more →
San Antonio is a city in the U.S. state of Texas. It is the seventh-most populous city in the United States, the second-most populous city in Texas, and the second-most populous city in the Southern U.S., with a population of 1.43 million at the 2020 census. 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.