These two phrases mean basically the same thing and are combined for emphasis. AT: "prevent the horses and mules from dying" (See: [[rc://en/ta/man/translate/figs-parallelism]])
The phrase "by himself" emphasizes that Ahab and Obadiah went in separate directions, not necessarily that no one was with Ahab. AT: "Ahab lead a team in one direction and Obadiah lead a team in the other direction" (See: [[rc://en/ta/man/translate/figs-synecdoche]])