Plex Documentation → Naming & Organizing Your Media → Naming and organizing your Movie files
See the Movies Split Across Multiple Files section.
If Plex has difficulty differentiating the Snyder cut from the original, add the IMDB ID to the folder and file names (see documentation for syntax). The two versions have different IMDB IDs, and Plex will use the ID to identify the correct version.
Consider combining the two parts into one movie. Not all Plex clients play nice with multi-part movies.
