I'm sorry to say, but it does not work.
I just tried to add a book with the same title/author (but different uid) and different language to the e-reader and the already existing book is +
replaced+ by the new one. So it seems that it is not possible to have +
both+ books present via calibre on the e-reader.
Of course, I can add the extra book outside calibre, but that is not what I want (missing collection info, etc.).
Possibly there is some workaround?