Reference for Chullin 161:28

קדשים שניהם בחוץ לרבנן ראשון ענוש כרת שני פסול ופטור מלאו דשחוטי חוץ

according to R'Simeon, he who slaughtered the second has transgressed a negative command.<span class="x" onmousemove="('comment',' Kareth, however, is not incurred, for since the slaughtering of the first was a valid and proper slaughtering the second was not fit to be offered this day within the Sanctuary.');">¹³</span>

Jastrow

Jastrow