नित्यद्विवचन समास पद​

[Sriram Nagesh Nookala]

Where can I find which compounds become नित्यद्विवचन? During my study of 
रघुवंशम्  I was told पार्वतीपरमेश्वरौ is a  नित्यद्विवचन.

I donot know of any source for that. Can someone point to some text that defines what makes something  नित्यद्विवचन ?,

thanks

Nagesh