Aristotelians who hold that soul and body are separable cannot be considered substance dualists according to their own account of substance since Aristotle explicitly distinguishes substance not only from accidents but also from the parts of substance (see parts 2 and 5) and he defines soul and body as parts of a hylomorphic composite.
For that matter, Plato can’t be considered a substance dualist since he doesn’t regard the body as substantial, but only as a fleeting accident projected on the screen of the spacial-void.
It is only in Descartes that we get soul and body divided as a res from a res, but even here it would be silly to think that Descartes thinks that both are equally substantial. It is very doubtful that he would ever say that matter has anything like what the self discovers in the cogito since it might exist but it is certainly not a self, and extension never suggests anything like the ontological concentration and self-setting-apart that one needs for a bona fide substance.
This is not just a dispute about language but points to an oversight in our accounts of substance dualism. If all we mean by “substance” in substance dualism is “what can exist separately” then everyone becomes a substance dualist about everything. We can donate organs, graft cells, tissues, and tree limbs, mix chemicals to form compounds, break apart the parts of atoms etc, and so everyone is a substance pluralist – even materialists. Again, any act of reproduction involves some part of a thing breaking off and attaining its own existence (this is true of both reproduction in the normal senses and of generating an identical twin. For that matter, it’s true of any cell division or DNA transcription) and so all these things should involve “substance dualism”. But this is probably an indication that we aren’t hitting what we’re targeting with the word “substance”, since even Descartes’s division of a res from a res isn’t enough to give us a duality of substance except in the sense that everyone accepts it.