I hold an american PPL which i was given on production of my UK PPL in 1993. Now you have to have your JAR PPL ratified by the CAA who give you a letter confirming your licence. My FAA PPL was therefore restricted in that it was only valid provided my UK PPL was valid. I am here in the US now having just completed the FAA CPL, they have given me a stand alone FAA CPL and new cert number (the restricted FAA PPL is now effectively pointless). So i hold 2 FAA certs with 2 different numbers. The only hassle is the securiy process and visa, but once here it is fine. You can add a multi add on to the CPL quite easily too.
Just my experience if it helps.
KK