When you plug two identical device (same VID, PID which also use the same port type like USB)... I don't see how an application can determine which hidrawX should be open to match another interface (like ALSA mixer).

I think a new IOCTL should be added which could be HIDIOCGPHYS and return the physical location just like EVIOCGPHYS in linux/input.h (/dev/input/eventX).