As I recall, the only time they crossed paths in canon was when Spike had Dru and Buffy both tied up and was trying to decide which one of them he was going to kill. (Harmony was definitely the fourth wheel there.) Each of them, Drusilla and Harmony, lives in a world that nobody else inhabits, so even what they have in common makes them utterly incompatible. It just seems that that fundamental conflict — both daffy, in entirely different ways — would provide lots of fun narrative possibilities.

Usually, if I’d really like to see a particular story, I go on and write it. This one, though, I wish someone else would do it so I could enjoy it from the outside.