Have someone in the first vehicle assigned to watch the second vehicle to catch any signals or signs of the second unit falling behind. Use one of Jan Noble's favorite items, duct tape, to spell out a HELP message on the hood or cab or roof of your vehicle. Planes are used for patrolling remote areas and spotting lost people and vehicles. Doesn't hurt to advertise you need help when you do, and vehicles are much more easily spotted than a person walking or down. Your vehicle can provide shade and should be carrying your survival gear too.
Water is first on the list.
Earthquake emergency response training in L.A. says 1 gallon per person per day should be available in case of an earthquake. In the desert, more is better. Take an extra stash of high protein type food bars beyond your day's lunch. It might mean the difference between a simple wait and a very hungry one. Have warm clothing and Mylar type survival blankets for cold night air.