For the naming of dual-file DSPs, they must share the same name plus "L" or "R" for the appropriate channel? Playing either file with vgmstream would be the same (assuming both files are in the same directory and are named correctly?)
There are a few other naming schemes, but L and R is one. Yes, playing either will be the same. They chose this scheme because Nintendo only provided a DSP encoder that worked with mono files. Rather than invent an interleave scheme (as everyone else did, and Retro did in Prime 2) they just used two files. You'll notice that in the demo they use ADP, which does support stereo, but it is quite a bit larger and one has less control in the playback. I guess they hadn't figured out quite what to do yet.
Get the rip here, that has all those tracks included and the problems (of volume nature in the demo files) corrected. If you want the original streamed files, you're out of luck as I'm too lazy to upload them right now. Maybe someone else will.
This is from the Metroid Prime set that includes both the real release and the trial one, so it's possible that redundant tracks from the real release are missing.