I know the FAA specifically prohibit this with their version of the "rendered valid".....if you have (say) a PPL SEP FAA full licence and a JAA with SEP and MEP, you cannot get a 61.75 "rendered valid" approval to fly MEP - though you CAN have the instrument rating privileges extended to a US licence.
Well actually you can. Say you hold an FAA CPL ME (standalone) and you have a JAA PPL with SE. Although you cannot hold two FAA certificates, you can actually have have the SE added as an endorsement on the FAA certificate as per FAR 61.75 process or whatevertheheck it is:
Commercial Pilot Multi Engine Land
Single Engine Private Privileges based on and only when acompanied by foreign licence number 1234567
yada yada yada.
My situation is reversed, I hold CPL ME SE IR in FAALand, yet in JAALand I am a PPL SEP. Still I can fly ME aircraft in JARLand because the CAA render it valid without formality. I believe I could also fly SET/ MET in JARLand should I wish to....