When using long exposure noise reduction the effect is burned into the raw file. You do not get separate files and I don't believe the light and dark frames are stored separately in the file. In fact I'm almost certain they are NOT since the files size is the same either way.

An alternative if you need the dark separate is to shoot it manually. Set long exposure noise reduction off. Then shoot another with exactly the same setup (exposure, ISO, etc) with the lens cap on. You can then do the subtraction in post processing. This is done with astrophotography all the time. Other than having access to the dark frame this has the advantage of being able to use one dark for many lights which saves time.

Note that for the best results the multiple darks should be averaged. This reduces noise in the dark that is not in the light frame.