Thanks Bernie & theducks!
VL is good using filter rule, I archieved previously with custom field.
But as theducks says, the root folder is the same.
I thought, maybe, if we can organize/customize the file structure per ....groups, i.e. folders per set of authors/titles subfolders... then in this case, I can use windows' mklink command to link the folders to somewhere else.
But this seems not possible currently. Hope for next release

(maybe not, it seems not to be the strategy of Calibre file structure)