That's only the produced sound, right? If the produced sound is at a frequency that coincides with the original sound (as opposed to being half a wavelength off), wouldn't it amplify the sound and make it up to 2x? Apologies if I misunderstood your comment.
I am not an audiologist... but I don’t think it would result in 2x the sound. I believe a sound originating from 2 sources with the same amplitude results in either phase cancellation or the same resulting amplitude, not 2x. This is similar to what happens with line arrays.
You are thinking about the volume. As another replier said, two interfering frequencies produce an interference wave equal to their difference, so you can't have supersonic waves from something only capable of producing 20kHz sound
