[JRTomlin]

I think Miss Fletcher is correct. Further, I think they don't keep a term for a two-part series because it's either one or two. One, two, three, four... is a series. One and two? Pair, couple, original + sequel, etc.

That being said, lots of people are using Duology and getting away with it. I bet it'll be a word in the dictionary in a few years.

I agree. I think the term duology is becoming more accepted and that it will be in dictionaries within a few years.

I'd say if you want to use the term duology, feel free. Otherwise it's a novel and a sequel.