Our Gemara entertains the possibility that a person is only Chayav a single Chatas, no matter how many of the Arayos he lived with (i.e. his sister and mother and aunt etc.). Later, the Gemara entertains a similar possibility; each of the Arayos obligate a unique Chatas except for adultery, which shares a Chatas with other Arayos.
How can we presume that a person is Chayav a single Chatas for living with two women (who are prohibited for different reasons)? Isn't there a rule (Kerisus 15a) that "Gufin Mechalkin" - that is, a person is Chayav a separate Chatas for each woman that he lives with, even if they are all prohibited for the same reason?
That is an excellent point.
The source brought in the Gemara to teach that a person is Chayav a unique Chatas for each individual woman, is the word "v'Isha" that is used by Nidah (Kerisus bottom of 9a). At this stage in the Gemara, we didn't yet know that Drasha (which is cited later on the page).
Thus, the Gemara's point here is indeed to bring a source for individual Chata'os for each person that a person sins with (Gufin Mechalkin), or for each prohibition that a person transgresses (Lavin/Kerisus Mechalkin). By default, we assume that one is Chayav only a single Chatas in all circumstances (since the Torah included all of the Arayos in a single Kares; see Shitah Mekubetzes #15).
Originally, the Gemara proves from "Achoso" that one brings a Chatas for each prohibition that he transgresses. Afterwards, the Gemara says this is not necessary. We already have a verse teaching that a person is Chayav an individual Chatas for each person that he sins with, so it is not necessary to teach that he brings a separate Chatas for every prohibition that he transgresses.
The Gemara answers that it is nonetheless necessary to teach that a person is Chayav a separate Chatas for each prohibition that he transgresses in the case of a single woman who is prohibited for a number of reasons.
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