The Fraser Sisters, Jo Freya and Fi Fraser. (Especially maybe when Ralph Jordan is playing for them.)
................

You do realise that, with a title like that, if this thread stretches out a little, we can expect to get companion threads "What's your favourite female diet?" and "What's your favourite female duvet?"

Actually, doesn't the term "duet" mean a piece for two sohers/musicians, while the term for two singers performing together is normally "duo". I get the impression that emily rain, in starting this thread, was in fact using it in the former sense.