When I first wrote Dark One's Mistress, it was to be a standalone. As I went through the edits, I knew there was more of Clara's story to be told. Which led to Dark One's Wife.
But since writing 50,000 words on Dark One's Wife in November (on top of the 25k I already had), I find myself in a peculiar state of mind. After much thinking I've come to the conclusion that my Duology may in fact need to grow into a Trilogy. At the moment, I've got the inciting incident happening at chapter ten which is halfway through the story, and that's with holding back all the stuff I want to show. The longer I think about it, the more rushed it feels, like I've only shown the highlights. Seems foolish as the first eleven chapters currently happen over a few days, how could it possibly be rushed? But there you go.
So the answer there is to write the little scenes that keep popping up whilst I deal with the whole terrifying mess of releasing two books on top of real life's woes. If applied, it would not only require massive rewrites (not bothered there), but will likely up the word count another 25k. That's not ever including what I'd need to write to get the last two chapters finished.
I guess that makes Dark One's Wife book three now, with Dark One's Bride being book two...