Wrong Side of the "Tracks"
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Getting from San Diego to Austin in two days is just within the range of a very purposeful, no frills, slog down the highway for a few good long haul drivers. You will have no time for anything but the barest food, fuel and bathroom breaks, and even then everything must fall in line - no traffic delays, no serious construction detours, no unexpected time outs, nothing untoward. You will also have to pace yourselves and stick to a very demanding schedule so that you can all get a solid eight hours of sleep overnight. Note especially, that is eight hours of sleep, not just eight hours off the road. Typically that means ten hours off the road including an hour to check into your motel, shower and wind down, and an hour in the morning to get up, packed, organized, fed and back up and driving. Note also that both of your days will only be 23 hours long as you lose an hour each day to time zone changes.
All of the above is by way of saying that aiming for "a few more hours past [El Paso]" is absolutely the wrong way to go. You want to cover no more than 650 miles or so on the first day, half way. That is already more than you should really attempt on a multi-day drive, but is just at the outer limit (being generous) of what you can accomplish with 2 to 3 drivers. Trying to cover more miles on the first day will leave you groggy in the evening and a danger to yourselves and others. And you will be totally exhausted the second day. NOT a recipe for a safe trip. You should plan to stop overnight in Las Cruces, NM. Use the "Hotel" tab in the green toolbar above to see what's available there and book ahead so that you waste no time looking around for something when you should be sleeping instead.
AZBuck