Calibre "second account"

Let me explain my wish:
I have a big library, both on PC and on reader. Everything is nicely organised and synced.
Every now and again, I need to convert a book for my daughters. A book I don't want in my library. This is what I do now: I import the book in Calibre, convert it, open the containing folder and copy it manually to daughter's reader, then manually delete it in Calibre so it won't sync with my reader.

How much easier would it be if I had a choice of Calibre "accounts". One account refers to my library on the PC, the other to a temporary library (which may or may not be empty) on PC. I don't have to worry about my library being polluted.

As far as I can tell, there is no optional choice of libraries Calibre refers to when starting up.
Adittionally, having 2 "accounts" (libraries) on hard disk may have all sorts of extra benefits. Maybe it would be nice to be able to merge them on the reader, while leaving them separate on PC.

But maybe all this has been thought of long ago and I simply overlook previous threads.