two books from the same content

1. copy the existing Book file and all Component .fm files to another directory at the same level in the filesystem hierarchy as the original book.
2. rename the new Book file as needed to distinguish it.
3. use the Book file to rename the copies of the component files as needed (may or may not be needed, and if it is, it's because you need to move the clone files back to the original dir for all imports to work).
4. in the clone files, delete all the Flow A content in all non-generated component files
5. in each such file, File > Import > File
   for the clone's file's original sibling in the original directory
   (*) By Reference
   then
   (*) Body Page Flow:
   (*) Retain Source's Formatting (typically)
   Updating (*) Automatic (well, we might wish it was).
   This is a Text Inset.
6. In the clone Book file, select all component files
   View > Show/Hide Conditional
   (*) Show as desired and [Set]
7. You now have two books, from a common source, with their own unique Conditional content display.

We do this. It works well where you are NOT using chapter-per-file, and have a single monolithic Body file. With multiple chapter files, chapter-to-chapter Xrefs might not work. Generated content (TOC, IX) should work however the books are structured. When in the clone book, you cannot edit the Text Insets, which is usually wise.

You could also, not having tried it, just set up a second Book file in the original dir, with a different name, and include/generate component files as needed. This would allow edit from either book.

This is a legacy way of doing this. With the Hierarchical Books feature of later FMs, there might be others.