The 1920 LDS edition changed the singular heart to the plural hearts in the LDS text since the context is plural (note the use of the plural our). Even so, the singular heart agrees with the preceding singular nouns thought and intent, the implication being that these Amlicites are of the same mind. A good example of a similar use of the singular heart in the Book of Mormon is found in a quotation from the King James Bible:
Note here the use of the singular mouth along with the original singular heart. But in Alma 21:6, one could argue that the singular nouns thought and intent led to an error in the early transmission of the text (namely, the replacement of an original hearts with the singular heart). Yet there is evidence in the original text that the singular heart can refer to a plurality, even in the immediate context of hearts or of a clear pronominal reference to hearts:
In the first example, the pronoun they is used to refer to an implied plural hearts, even though the preceding text originally reads “your heart is not drawn out”; in the second example, we get both a singular and a plural form of heart within the same passage. In both these examples, we see the tendency in the history of the text to replace the singular heart with the plural hearts. In general, the critical text will restore instances of singular heart with plural reference whenever they are supported by the earliest textual sources.
There are two other cases of a plural context where the original text may have had the singular heart:
For discussion, see under these two passages.
Summary: Restore in Alma 21:6 the singular heart, the earliest reading; the Amlicites are here characterized as being one in “thought and intent”, which is consistent with the use of the singular heart.