When I wrote my first book, I kept writing the story (newbie mistake) and it was waaaay to long, even after major trimming. It is even a long story to tell why it happened that way. But it was all one cohesive story. Book 1 depends on Book 2 for the finish, so I published them both at the same time as:
The Roaring Road with the subtitle (clearly indicated on the cover) Book 1 The Road West
The Roaring Road with the subtitle Book 2 The Road East
They are not standalone books, it is all one novel and I have promoted them that way. The covers can be seen on the book website, link in my signature below. The title is very clearly the large name, the subtitles appear smaller in the right lower front corner and on the spine of the paperbacks. All the retail outlets list the books properly, only GR feels they are above the law when it comes to ISBNs and the author's intellectual property.
Since I have story plans for a six book series I named the series The Roaring Road right from the start, and listed the books as a series on the ISBN numbers. The first books were meant to be the leaders of the series. The librarian moderator changed the titles to:
The Road West, The Roaring Road series Book 1
The Road East, The Roaring Road series Book 2
The librarian manual says:
"An exception to this is when the series name is the primary or only title. In this case, the title may be formatted with series name and number as the title and any additional title text as the subtitle."
The third book is Road Trip Blues with the subtitle Book 3 of The Roaring Road Series
At least they didn't mess that one up.
With perfect hindsight I could have given Book 2 a completely different title, but that may not have helped librarian reading comprehension.
but my beta readers and people who have read the books have no trouble comprehending what the titles mean. I never had to explain it, as it is quite obvious to readers, maybe not so much to GR librarians. BTW I sold a lot more paperbacks than were sold on Amazon, so I had a lot of direct reader feedback, from people I sold the books to and never saw again, and those who delightfully gave me a running commentary as they read the books ("how will Laure get out of this?" was a common question).
I am (at least for now, who knows what GR will do to my status since I'm raising hell about this) a librarian since authors can be librarians if they ask for it, which I did two years ago. I never heard anything about it being time-limited.