On Page 1, Part 2, it says "Information on the Principal Immigrant." That's pretty clearly referring to the principal immigrant (not the petitioner).
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The short answer is that Part 2 refers to the principal immigrant. The better answer is that Form I-864 is complicated and it is worth hiring an experienced immigration attorney to help you. If the cost is a problem, there are some attorneys and some organizations in the northern Virginia / D.C. area who offer immigration help at low cost.
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