Reference for Makkot 27:6

פשיטא הרי גופין מוחלקין הרי שמות מוחלקין אלא לחייבו על אחותו שהיא אחות אביו שהיא אחות אמו והיכי משכחת לה ברשיעא בר רשיעא

This should be obvious!? They are separate bodies and they are separate prohibitions, [why do the Rabbis need a verse to teach us this?] Rather to make one liable [separate offerings for cohabiting with] his sister who is his father's sister, and who is [also] his mother's sister. And how can we find such a case, with a sinner the son of a sinner [re'shia bar re'shia. cf. Rashi].

Jastrow