Thanks BetterRed. You're right, it will not work if we do it at book title/author level. That's why I mean to create a link per group of books, not per book, and that works, as long as the link is at the root folder/library level, not at the book level.
Currently, this solution works for me only per library, which I can, for instance, displaying 2 libraries at the same time, one with main instance of Calibre, and another with a 2nd Calibre via VMWare, pointing to libraries via either links or shares.
If we cannot have a folder per group, maybe in a future release of Calibre, there would be a VL which is not only a subset/projection of a library, but also a joint view/union of libraries?

Is that possible, Goyal?