If, in a question I've asked, it transpires that a secondary question needs to be asked/answered in order to adequately answer the primary question, how/where should I ask that secondary question?
Should I edit the main question to include the secondary question?
Should I comment on my main question in order to ask the secondary question?
Should I post it as a separate question?
The general advice is to edit your original question only if you're clarifying a point for that specific question (or to fix spelling/grammar/tags/etc).
A new question, even if it's somewhat/wholly related to another, deserves its own post. That way both questions have a distinct focus, can be useful to future visitors, will attract the specific niche expertise required to answer them, and won't get muddied with answers/comments that refer to (potentially) different disciplines.
Referring back to the original question in the new one, by way of a link, is probably the best thing to do; to make sure the new question stands on its own, I'd include some of the context from the original too. Not all of it, of course, but enough to give users who haven't seen the original question a chance to understand what you need.
Should I edit the main question to include the secondary question?
If your question doesn't have any answer, then you can edit the question to add the secondary question, supposing that the secondary question is very minimal compared to the main question; if answering the sub-question requires to write a longer answer than the answer for the main question, then write it as a separated question that references the first question you are asking, if that helps the users who answer the question.
Once the question has answers, you should not edit it in a way that changes its meaning. That would invalidate any given answer.
Should I comment on my main question in order to ask the secondary question?
Comments are not for asking secondary questions, except in the case you are asking what a user meant in a comment written for your question.
