פירוש על עבודה זרה 97:8

The problem with the above suggestion is that it depends on Abaye’s explanation of R. Eliezer. Abaye is the one who implied that in cases of two causes R. Eliezer prohibits. Abaye’s explanation allows us to pair R. Eliezer and the sages dispute with the two baraitot concerning the field fertilized with idolatry or cow fed with idolatry. But the simple reading of R. Eliezer is that the ruling depends on which substance is put in last. If the permitted leaven was put in last, R. Eliezer may permit the dough even though the forbidden dough was still present. In other words, R. Eliezer may not hold that in all cases of two causes the result is prohibited. He may hold that the principle is that we simply follow whatever was put in last.