Hi Nal,
You are right, the FDA does specifically mention Phen with Fen.
It was the below message that caught my attention:
"there have been reports that physicians are prescribing phendimetrazine in combination with phentermine in a limited number of cases. We emphasize that such concomitant use is not indicated."
I can see Phen with Prozac or Effexor, they do different things. But Phen with Phendimetrazine? Seems like stimulant overkill to me.
But I am not a doctor, I just play one on the internet.