To have a "second sponsor" on an I-134 form, which is the Declaration of Financial Support, you essentially need to have a "joint sponsor" by including another individual's information on the same I-134 form when you are not able to meet the income requirements alone; this is typically done when the primary sponsor's income is not sufficient to support the beneficiary based on federal poverty guidelines; both individuals would need to provide their financial information on the form to demonstrate their combined ability to support the beneficiary.
Key points about adding a joint sponsor on an I-134:
One form, multiple sponsors:
You do not file separate I-134 forms for each sponsor; you simply include the details of the joint sponsor on the same form.