Oh, thank you for your answer, even if it doesn’t help me with my observations and needs.
I’d be glad to have them as series, but TheMovieDB holds them as movies already and they do not accept changes for this (while other multi-part movies are treated as series at the same time).
Maybe there’s some command line magic to really separate a certain object in a certain library and give it its own stati?
I’d also be happy to understand the “split apart” process and its use cases?
I currently have movies matched as duplicates (which then I feel the need to split apart) in the following situations:
a) mistaken identity
Due to naming errors or similarities with titles, a movie gets false identification as a movie that’s already in my library.
Analysis: These are two movies that don’t have anything to do with each other.
Solution: Split them apart and do a manual identification (preferred since it keeps the Watched/Unwatched status of the original movie), or fix the naming and Plex Dance these items.
b) real duplicate
With older movies, it happens now and then, that these movies are known under different titles, but are essentially the same movie. I appreciate that Plex shows them as duplicates.
Analysis/solution: Check the titles with Wiki/IMDB, decide which one to keep and delete the other one, thank Plex for keeping my library free from Duplicates 
c) different cuts
There’s a reason why collectors have different cuts of the same movie. Maybe one is acceptable for the youth, the other is for mom and dad. Or both have unique scenes in it, or just for completeness…
Analysis: Since I keep both of them I want to watch either of these. They may have different “content ratings” so one wants to split them apart, so you can edit the titles to … (unrated) or something for better distinction.
Solution: Different cuts with different lengths share the same progress indicator, watched/unwatched? Come on. I cannot imagine stopping to watch the rated movie at a certain point and continue to watch the unrated movie then. I wouldn’t say that there’s no sense in it at all, but it wouldn’t be common use case, I suppose. Oh, and did I already watch the unrated or the extended version of a movie? Cannot tell, since they share the same watched status. Option for real separation would be perfect.
d) multi-part movies
Full-length movies of 90 minutes bound together as a mini series… I normally throw them to my series library. But some of these (see my original posting) need to be handled as movies since the TMDB only provides data for these as a movie.
Analysis: Having these split apart and handled as separate movies (renamed to … (part one) etc) would be perfect for these.
Solution: I could stack them, creating big movizillas of a total length of 3 or 6 hours. Technically possible, I feel uncomfortable with this situation. I could explain why as this is for more than one reason, but I let you escape this time 
Did I miss another possible scenario why one wants to split Duplicates?
I can hardly imagine a common use case where keeping these separated movie objects makes sense and I tried to explain why.
But maybe I miss the total point of this?
First reaction if I were to program this, would possibly be “Oh, lot of work, better not touch it. Needs addition to UI (if button solution), we cannot foresee side effects.”
But maybe I am just too focussed to my own advantage/workload as a programmer to see why I function that I thought was fine, does not fulfill the all-day task very good.
Is there a Plex team member, a Plex ninja who wants to help me understand the split apart function and what it is intended to do (plus, why is shared progress, watched status a good thing)?
I am not trying to make your life harder. I really want to learn. Or maybe we can come together to new conclusions of what would be a fine solution? Wouldn’t that be cool?
Thank you for listening.