The fair place to meet is Las Vegas, NV — the city closest to the midpoint of Boise and Salt Lake City. From the farther side that’s about 10 hr 51 min of driving.
Recommended midpoint
Las Vegas, NV
From Boise
10 hr 51 min
517 mi to Las Vegas
From Salt Lake City
7 hr 36 min
363 mi to Las Vegas
Salt Lake City has the shorter trip; the split is off by about 3 hr 15 min. The alternatives below can even it out. Boise and Salt Lake City are about 296 miles apart.
Boise and Salt Lake City are about 296 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 10 hr 51 min of driving on the Boise side and 7 hr 36 min on the Salt Lake City side — the fairest single meeting point among the cities near the middle.
At this range, Las Vegas works for a same-day meetup: close enough from both Boise and Salt Lake City to meet for lunch or an afternoon and still be home by evening.
If Las Vegas doesn't have what you're after, Sacramento and Portland are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Boise 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 →
Boise is the capital and most populous city in the U.S. state of Idaho. It is the county seat of Ada County. The population of the city was 235,685 at the 2020 census. 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.