Reference for Keritot 29:19

הבא על אחות אביו שהיא אחות אמו חייב אחת על כולן או חייב על כל אחת ואחת מהו

- This is rather what it means to state: What [is the law] if one had intercourse with a sister who is at the same time his father's sister and his mother's siste is he liable to one sacrifice for all the trespasses, or to one [separate] sacrifice for each of them?

Jastrow