What is ชุดนิยายสองเล่ม?

A duology has exactly two books that form one cohesive story or closely linked pair, whereas a trilogy has three and often supports longer, more layered plotting. A standalone is a single book with a complete arc. Duologies sit between the two: more room than a standalone, but less sprawling than a trilogy.

Sometimes. Some duologies are written as two halves of one continuous plot that works best read in order; others are paired tales where each book can be enjoyed separately but gains extra meaning when read together. Marketing and blurbs usually indicate which approach has been taken.

Duologies are fairly common, especially for authors wanting to expand a hero/heroine’s arc without committing to a long series. In romance, readers generally expect a satisfying emotional resolution (HEA or HFN—happily ever after / happily for now) by the conclusion of the second book, even if the first book leaves things unresolved.