From the Epistle of the Same Nicholas, to Salomon, Bishop of Constance, A.D. 867

Your holiness asks of us whether some man can have two co-mothers, one after another. In this he ought to remember that it is written: they shall be two in one flesh. Therefore, since it is established that man and woman are made one flesh through marriage, it remains, namely, that the man is constituted co-father to that woman whose co-mother the wife assumed in marriage appeared to be. And for this reason it is clear that a man cannot be joined in copula to that woman who was co-mother of her with whom the same man had been made one flesh.

The last things are clear: the spiritual cognation contracted by the wife flowed back into the husband.