This verse quotes Isaiah 29:13 so our discussion of original language compares the original Hebrew and the resulting Greek. The Greek of the Septuagint is identical to Christ's Greek. Notice that the "heart" is singular for the plural "theirs," one heart shared by the group. This is typical of Jesus, indicate that he see "the heart" as a shared feeling. (See this article on Jesus's special use of this word and related words). Note that the Septuagint uses the word in singular as well.
Christ's Words in Matthew as a Guide to 40 Days of Prayer.