I don't think that quarto book does have such feature yet. Each document is independent which is closer to KM approach (but there is not merge really).
Best place to ask question and open a discussion for considering new feature or just discussion current behavior is at
Quarto team closely follow the discussion board and that is why this is considered the best place for now.
I let you transfer the question there, ok ?
That would be interesting to have a discussion around it probably. Probably some project related feature could help with this usage. We'll definitely want to know possible limitations from bookdown behavior for R users. thanks!