I'm not sure if this will do what you are looking for, but I use it for
my one keyboard.  It remaps any keys that return a scancode (some don't
like those pesky Fn keys on laptops).  If the keys on the USB keyboard
return a different scancode from those of the PS/2 keyboard, this
should work.  If they don't then someone more knowledgeable than me
hopefully can answer.

http://www.freewareweb.com/cgi-bin/archive.cgi?ID=1698

My suspicion is that they do return the same scancode.  Which would be
unfortunate.  I'd try it myself and let you know for sure, but I don't
have two keyboards.  If you just want it for media controls, there are
plenty of cheap keyboards out there with those built in.  

Sean