Although if both in physical form one could slay the others physical form, whilst the spirit wouldn't die then it would have at least been a Victory per se. Didn't Tulkas fight with Melkor before he took the Simarils "whilst the earth was still young" why fight if it would lead nowhere the physical form can be slain even if they were to take a new physical form.
http://www.glyphweb.com/arda/m/morgoth.html
That just shows the translation of the name Morgoth and yes it sounds sligtly racist but I think Tolkien means it more in respect of Black and White, Negative and Positive, Cats and Dogs and so forth rather than actual colouration of said beings.