The original text had two occurrences of “a true and a living God” in this passage. But since this construction with its repeated a is not normal English, the tendency has been to remove the repeated indefinite article a. For the first example, the 1837 edition accidentally deleted the second a, an omission that all subsequent editions have followed; for the second example, it was the 1841 British edition that deleted the repeated a, and all LDS editions have followed that reading.
A parallel construction, “the true and (the) living God”, involves the definite article the. For this construction, there are two instances in the text of the repeated the (Alma 7:6 and Alma 43:10); but there are four instances where the the is not repeated (1 Nephi 17:30, Alma 5:13, Alma 11:25, and Mormon 9:28). Note that one of the nonrepeating instances occurs here in Alma 11:
The lack of the repeated the in verse 25 may have contributed to the omission of the repeated a in the two instances of “a true and a living God” found in the immediately following verses 26 and 27. The critical text will maintain the original variation: in each case we follow the earliest textual sources in determining whether the article (either the indefinite a /an or the definite the) should be repeated in conjunctive constructions. For further discussion, see under conjunctive repetition in volume 3.
Summary: In accord with the reading of the earliest textual sources, restore in Alma 11:26–27 the repeated indefinite article a for both occurrences of “a true and a living God”.