I agree with OP's point: the word synchronize implies that two things begin to share some common value. In this case, the heart rates share similar behavior (no

I agree with OP's point: the word synchronize implies that two things begin to share some common value. In this case, the heart rates share similar behavior (not ~exact BPMs), but I think most folks wouldn't choose the words used to describe this. A more apt title might say "the changes in their heart rates synchronize".