Here the 1830 typesetter accidentally dropped the repeated his. Elsewhere the Book of Mormon text always repeats the determiner in conjuncts involving horses and chariots, including two examples in the next few verses:
For additional discussion of the tendency to omit the repeated determiner in conjunctive structures, see under conjunctive repetition in volume 3.
Summary: Restore the repeated his before chariots in Alma 18:9; this repetition of the determiner is in agreement with all other conjoined examples of horses and chariots in the text.