I have combined my movies without issues. I have my mount point media/movies within this folder I mount my 4k and 1080p movie libraries. I have everything broken into folders within the 4k and 1080p shares by Movie Name (Year)/Movie Name (Year) - Quality. Plex scans this and adds movies correctly. It does a great job at selecting quality as well.
I have tried this with TV Shows and Plex ends up adding everything into one or two shows.
I have a mount point of media/TV
Within the TV mount point I mount 4k and 1080p shares.
Within these shares it is broken out with TV Show/Season/TV episode - quality.
No additional folder levels, nor fewer levels.
Your library in Plex must point exactly to the folder wherein all the folders of your different tv shows are stored.
/TV Shows <-- library points here
/Doctor Who (1963) {tvdb-76107}
/Season 01
S01e01 - An Unearthly Child (1).mp4
S01e02 - The Cave of Skulls (2).mp4
No, you can easily point a Plex library to two or more folders. No need to play around with mountpoints.
So point it to the folder with the FHD versions and additionally to the folder with the 4K versions.
(the screenshot is from Windows, but it doesn’t matter. What’s important is the ability to link a library to more than one path)