The help centre’s What types of questions should I avoid asking? is linked in many prominent places guiding users on how to ask questions. However, with respect to too broad question contains only the following line:
Your questions should be reasonably scoped. If you can imagine an entire book that answers your question, you’re asking too much.
This fails to capture the case that a question asks multiple questions (that are sufficiently independent to be asked separately) — which is also bad and explicitly closeworthy but does not necessarily mean that an entire book is needed to answer the questions.
As this is a common problem, I propose that a respective sentence is added to address this issue, e.g., something along the lines of:
Also do not ask multiple questions at once unless you expect their answers to overlap considerably.