[thaumasiotes]

> Is there a precedent for this with other disorders? Seems to me that if there are no overlapping symptoms, it should be a separate disorder.

Yes, this is a pretty normal way of doing diagnoses.

Specifically, many disorders are diagnosed according to the template "the disorder is present if the patient presents any X out of this list of Y symptoms". As long as X is less than half of Y, it's possible for two patients to "have" the disorder without sharing any symptoms.

"Have" is in scare quotes because, obviously, this state of affairs is an artifact of the diagnostic criterion. However, it's also possible for e.g. two people to host infections of HIV without sharing any symptoms -- one may have AIDS while the other is asymptomatic.