Transfering books from one Calibre library to another

I just can't find a way to transfer a selection of books from one Calibre lib to another.

I can't use export/import, this not about exporting the entire lib.
I obvioulsy want to avoid having to re-import file by file.

i just want to move the books as they with their multiple versions (epub, moby) and find them exactly as they were in the other lib.

thanks for any help