The affidavit of support form is a very complex and confusing form, as you are discovering. However, I believe your confusion with page one, part 3, item 8 is that the "applies to cases with two joint sponsors" is applicable only to a "no" answer, not to the entire question.
Thus, a petitioner filing for his/her relative generally would mark "yes" since that person is sponsoring the principal immigrant. (A co-sponsor who may be sponsoring someone else's child, but not the child's parent would be a scenario for the "no" answer.)
Certainly, it can be helpful to have the assistance of a licensed, experienced immigration attorney to ensure forms are completed properly and that there is sufficient supporting documentation. Mistakes are easy to make, and can result in very serious consequences, including denial and referral for removal (deportation) proceedings.
If you need assistance locating experienced immigration attorneys, you can look here on Avvo, at www.aila.org, www.ImmigrationLawHelp.org, or http://www.justice.gov/eoir/probono/states.htm.
Ms. Doerrie's answer to your question is general in nature, as not all facts and circumstances relating to the specific person(s) and situations involved are known to her. Ms. Doerrie recommends consulting with an immigration attorney regarding your specific facts and circumstances prior to making any legal decision or submitting any form or application. This response does not create an attorney/client relationship.
I agree with Atty Doerrie.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.