El Paso and San Francisco are about 993 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 12 hr 12 min of driving on the El Paso side and 8 hr 44 min on the San Francisco 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. Las Vegas still makes a fair, central place for El Paso and San Francisco to converge, splitting the travel instead of asking one side to cross the country.
If Las Vegas doesn't have what you're after, Phoenix and San Diego are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between El Paso and San Francisco.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.