Matching a miniseries from a BD that has all episodes combined into one file?

Plex Media Server Metadata & Adding Files
2

If you want to keep it as one file, the correct naming should be

TV Shows  <- the folder linked to your tv-show library
  Salem's Lot (1979)
    Season 01
      Salem's Lot (1979) - s01e01-e02.ext

Alternatively you could attempt to split the original file into the actual 2 episodes.