Austin and El Paso are about 528 miles apart by road. Split the difference and you arrive near San Antonio, the city closest to the halfway point between them. That puts roughly 1 hr 50 min of driving on the Austin side and 10 hr 31 min on the El Paso side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so San Antonio suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If San Antonio doesn't have what you're after, Dallas and Albuquerque are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Austin and El Paso.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.