tayor iti dravyaguṇatvādijātyoḥ / tataścābhinnanimittatvāt tatsaṅkaraprasaṅgo durnivāra iti bhāvaḥ //
"Of both" refers to the universals of substance and quality etc. Therefore, because of having an undifferentiated cause, the unwanted consequence of their mixing is unavoidable - this is the meaning.