The kohen is to offer the one for a sin offering, and the other for a burnt offering. So the kohen will make atonement for her before Adonai for the uncleanness of her discharge.
“But if one cannot afford a lamb, then he shall bring, as his trespass offering for the sin he committed, two turtledoves or two young pigeons, to Adonai. One is for a sin offering and the other for a burnt offering.