Everything I can find indicates that an athlete who has already used the one-time transfer exemption has a tough bar to hurdle in order to get another post-graduate transfer exemption.
The other permissible exemptions involve non-recruited/non-scholarship athletes, family hardships, discontinued programs, military service, or a couple more obscure exemptions which wouldn't apply to most student-athletes.
It seems that most post-grads looking for a 2nd transfer would need to go through the NCAA Waiver process. And, it seems this waiver process is weighted against allowing the transfer unless the current school does not provide a grad program in the students field.
Of course this could all be wrong, but that's just the way I'm reading the documentation on the current NCAA rules. Maybe they're being more lenient on the Non-Scholarship Transfer Exception that Rick mentioned above, or maybe they're just being more generous on the waiver guidelines?