In this passage scribe 2 of 𝓟 accidentally copied the original singular heart as the impossible hearts, undoubtedly because of the preceding plural nouns thoughts and intents. Oliver Cowdery corrected the text to the singular heart when he proofed 𝓟 against 𝓞 (which is not extant here).
Summary: Maintain in Alma 12:7 the corrected reading in 𝓟, the singular heart in “they knew the thoughts and intents of his heart”.