Okay, here’s how I got it to work as wanted.
First, I changed the file names to this.
This gave me the correct custom artwork for both including different fanart and different posters. This did not produce any textual metadata. Then by doing an Edit on each and selecting the correct movie in the edit they both had their respective textual data.
Then I play a portion of the first one and stopped. The other movie did not show any progress and could be watched at a different time.
If you don’t need to have your custom artwork you can just have this.
and both movies are independent after you do the edit and select the correct movie.
Note the different watch status on both.
Edit to add:
Here are the pre-playback screens for both, note the artwork and fanart (background) are correct as is the synopsis and watch status.