Im going to try
If you are joint sponsor for a USC who is petitioning for the petitioners parents. These are standalone petitions. A separate petition for each parent .. a separate I864 for each parent Are you submitting two I 864 forms ..one for each beneficiary ? or are you joint sponsor for only one ?
As each application is stands alone , your household will be 2+1 immigrant on each one. So household of 3 on each form. As the time on completing the form, you are not sinusitis either of them .. So you do not include the other parent on the household number.
If your income alone is sufficient, you put your income on the form, submitting the IRS tax transcript and your W2 plus pay stubs as documentation. Your income for the previous tax years can be demonstrates by your w2. Your current year income is documented my your payslips