In the Book of Mormon text, the indefinite article is often repeated when attributive adjectives are conjoined (as originally in Omni 1:28: “a strong and a mighty man”). But in this instance from 1 Nephi 11:35, Oliver Cowdery accidentally added the repeated a when he copied the text from 𝓞 into 𝓟. Nonetheless, the 1830 compositor omitted the repeated a even though he apparently had no access here to the reading in 𝓞.
Elsewhere in the text, no initial determiner or other premodifier is ever repeated when an adjective is conjoined with the word spacious:
For examples involving other adjectives, see conjunctive repetition in volume 3.
(“a large and spacious building”).
Summary: Maintain the original reading in 1 Nephi 11:35 without the repeated indefinite article a