Questions: One I-539 for two people?

I am going to file I-539 extension for my parents. Seems I just need to file one I-539 for them. Is this correct? I wonder the form asks chief applicant lots of Yes/No security check questions in the form. But in the supplement page, there is no security question for the other applicant.

--Is this normal?

--Should I fill another I-539 for my Mom? Also if I file for two people, how much money, I need to include, just $300?

--On the form, PART 3, question 3, it asks "Is this application based on a separate petition or application to give your spouse, child, or parent an extension or change of status?"
Should I say "No" or "yes, filed with this I-539"?

Thanks!