You just need a complete “thought” for each book. For instance, I expect my current wip to be a two book series:
In book 1, the royal family is murdered and the king’s youngest brother takes the throne, becoming a tyrant and executing his political enemies. By the end of the book, it’s revealed that the prior king’s teenage son survived the assassination, and is the legitimate heir to the throne.
In book 2, he masses an army and declares war on his uncle to retake the throne. Technically I could put all of that in one book. Likewise, Tolkein could’ve put all three LoTR books into one volume too. But that one book would weigh a ton! (>‿◠)
Are one and two something that you can have cause and effect for? IE the healer murder’s their mentor and then answers the call for the doctor for the founder as a way to escape justice for the crime?
an organization needs a healer; sends for the mentor (probably sends for all the other advanced healers too but that’s not important rn)
the young healer murders the mentor before the messengers arrive
when the messengers arrive she is mistaken for the healer they’re supposed to retrieve (her mentor) and she lies that she is her mentor so they will take her away before her crime is discovered. she must keep up this lie (at least until she has made herself invaluable or found a different escape plan), or be killed for murdercrimes!
edit: that is only one book worth of plot though, imo. well, like the first arc of a book.