The three-year rule of which you speak is to determine whether a patient is new or established. It does not apply here.
90791, Psychiatric Diagnostic Evaluation (without medical services) does not designate new or established. It is not a "new patient visit", although a lot of mental health offices may call it that. The CPT chapter states, "an integrated biopsychosocial assessment, including history, mental status, and recommendations."
The credentials of the provider really don't come into play here; however, I would certainly question why two separate providers that practice in the same group BOTH need to do an evaluation. What would be more common would be for one to do an evaluation, come up with a treatment plan, and then, for any number of reasons, another provider in the group does the psychotherapy (90832-90837). I could see going to one provider, having the eval done, and then not really wanting to see them again and going to a different provider in a different group, who would then do another evaluation. But two providers in the same group? I would think it would be difficult to justify.