like they are two different things but have the same purpose. Which is to help the peitioner reach the 125% poverty line. A co-sponsor (resides in the same household as the petitioner) fills out the I-864A. A joint sponsor (doesn't reside in the same household as the petitioner) fills out another I-864. But no matte what, the petitioner still needs to fill out an I-864 whether or not he or she meets the 125% poverty requirements. God bless!