Reference for Chullin 164:11

שחטה ואת בת בתה [וכו']:

To teach you two things.<span class="x" onmousemove="('comment',' First that the prohibition applies where the animals were slaughtered by two persons, and secondly that whichever was slaughtered first, with the slaughtering of the second the law is infringed.');">³</span>

Jastrow