Lots of animals, e.g. dogs, can hear sounds beyond the frequencies that humans can hear.
Would it be realistic to have a basically humanoid species which had two sets of vocal cords - one similar to a human's and another that created high frequency sounds in the frequency range of a dog whistle in the nasal passage, together with an expanded frequency range of hearing?
The notion is that an ability to use a nasal dog whistle-like voice box to communicate at frequencies that humans can't hear would look like telepathy to a casual human observer.
A related question.
If a humanoid like creature had this second voice box and range of hearing adaptations, would it need further adaptations (e.g. an additional set of ears or an ability to focus individual ears on different frequencies) to communicate in both the ordinary and ultrasonic channels simultaneously? Or, would the second voice box and additional range of frequency sensitivity be sufficient by themselves to carry on two simultaneous channels of communication?