In order for any Post-Doc hours to count (in my state of residence) the State Board needs to approve the supervisor or none of the hours will count.
So technically they could be supervised by the Psychologists in our practice (yes, the ones with way less experience!) and then 1 or 2 years later sit for their Psych. boards. If this were done they would need supervisor approval by the state.
But the question then becomes, why would they do that if they are already licensed to practice? Guess there is a bit of ego in everything.