Thus Lateran in 649 (against the Monothelites), canon ii, “the Word of God with the flesh assumed by Him and animated with an intellectual principle shall come . . .”; Vienne, 1311-12, “whoever shall hereafter dare to assert, maintain, or pertinaciously hold that the rational or intellectual soul is not per se and essentially the form of the human body, is to be regarded as a heretic”; Decree of Leo X, in V Lateran, Bull “Apostolici Regiminis”, 1513, “. . . with the approval of this sacred council we condemn all who assert that the intellectual soul is mortal or is the same in all men . . . for the soul is not only really and essentially the form of the human body, but is also immortal; and the number of souls has been and is to be multiplied according as the number of bodies is multiplied”; Brief “Eximiam tuam” of Pius IX to Cardinal de Geissel, 15 June, 1857, condemning the error of Günther, says: “the rational soul is per se the true and immediate form of the body”.
From the above found in the Catholic Encyclopedia, it is difficult to see how anything like the " Dualism " of Descartes could be held by Catholics. I would not say such a position is condemned by the Church according to Anathema cited above, only that it is difficult to see how one could believe both that the soul is the form of the body but at the same time the body could be one substance and the soul another.
We also have the following from the Catechism:
II. “BODY AND SOUL BUT TRULY ONE”
362 The human person, created in the image of God, is a being at once corporeal and spiritual. the biblical account expresses this reality in symbolic language when it affirms that "then the LORD God formed man of dust from the ground, and breathed into his nostrils the breath of life; and man became a living being."229 Man, whole and entire, is therefore willed by God.
363 In Sacred Scripture the term “soul” often refers to human life or the entire human person.230 But “soul” also refers to the innermost aspect of man, that which is of greatest value in him,231 that by which he is most especially in God’s image: “soul” signifies the spiritual principle in man.
364 The human body shares in the dignity of “the image of God”: it is a human body precisely because it is animated by a spiritual soul, and it is the whole human person that is intended to become, in the body of Christ, a temple of the Spirit:232
Man, though made of body and soul, is a unity. Through his very bodily condition he sums up in himself the elements of the material world. Through him they are thus brought to their highest perfection and can raise their voice in praise freely given to the Creator. For this reason man may not despise his bodily life. Rather he is obliged to regard his body as good and to hold it in honour since God has created it and will raise it up on the last day 233
365 The unity of soul and body is so profound that one has to consider the soul to be the “form” of the body:234 i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather their union forms a single nature.
So while " Dualism " may not be condemned, it is difficult to see how a Catholic can regard it a valid/licit view and still hold to what is taught by the Catholic Church. I would say that those who hold to some form of " Dualism " must be able to justify it by some positive teaching by the Church. And I don’t see how that is possible.
Those who maintain such a position are faced with the problem reconciling the positive teaching of the Church and not being able to support their position by reference to a positive teaching of the Church. It would appear to be impossible.