I take it that you haven’t got a timeline yet, and are trying to figure out how to implement on just using Scrivener? I’m also assuming that you have combined the two books into one project for the sole purpose of figuring out your timeline, and not for future submissions/publications/revisions. If either of these are incorrect then the next advice is probably not going to be a good idea to follow.
First, you need to store your “time data” in a way that you can sort. That means you can put the date into the start of the title of each chapter, or you can create a custom metadata column to store this information in. If you are using real-world dates, create a date-type custom metadata column (not a text field). If you are using a non-standard date, then a text field will do–but you must make sure it will sort alphabetically.
Example non-standard dates that will sort:
2019-01-day-01
2019-01-day-19
Year 03,Day 001
Year 10,Day 100
Before proceeding, you probably want to be able to easily distinguish which documents were part of which book. You can create keywords for each book you’re working on, or use Labels (which are useful for some cork board views), or status, or even another metadata column (I sugged the list type, so you can just pick which book it came from, or that it’s a note, or whatever other kinds of documents you plan to create).
Once you’ve populated this data into your metadata, then you can load your books into the outline. The documents must all occupy the same top-level folder (so if you have them in folders for Book 1 and Book 2, you’ll need to drag all the chapter documents out into the Manuscript folder.
Then load everything into outline view, and sort on the date field (after enabling it in the outline view, of course).
Now you can add new documents in the timeline where you want to add new story elements. If they’re just notes, you can mark them as such using metadata as described above.