The fair place to meet is Las Vegas, NV — the city closest to the midpoint of Phoenix and Salt Lake City. From the farther side that’s about 7 hr 36 min of driving.
Recommended midpoint
Las Vegas, NV
From Phoenix
5 hr 22 min
256 mi to Las Vegas
From Salt Lake City
7 hr 36 min
363 mi to Las Vegas
Phoenix has the shorter trip; the split is off by about 2 hr 14 min. The alternatives below can even it out. Phoenix and Salt Lake City are about 505 miles apart.
Phoenix and Salt Lake City are about 505 miles apart by road. Split the difference and you arrive near Las Vegas, the city closest to the halfway point between them. That puts roughly 5 hr 22 min of driving on the Phoenix side and 7 hr 36 min on the Salt Lake City side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Las Vegas suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Las Vegas 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 Salt Lake City.
Las Vegas, colloquially shortened to Vegas, is the most populous city in the U.S. state of Nevada and the county seat of Clark County. It is the 24th-most populous city in the United States, with 641,903 residents 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 →
Salt Lake City, often shortened to Salt Lake or SLC, is the capital and most populous city of the U.S. state of Utah. It is the county seat of Salt Lake County, the most populous county in the state. 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.