Mishnayos Chullin Perek 2 Mishnah 2

With regard to one who slaughters by cutting two animals’ heads simultaneously, his slaughter is valid. If two people are grasping a knife and slaughtering one animal, even if each is holding a knife and slaughtering one above and one below, with each one slaughtering at a different point in the neck, their slaughter is valid.

הַשּׁוֹחֵט שְׁנֵי רָאשִׁין כְּאֶחָד, שְׁחִיטָתוֹ כְשֵׁרָה. שְׁנַיִם אוֹחֲזִין בַּסַּכִּין וְשׁוֹחֲטִין, אֲפִלּוּ אֶחָד לְמַעְלָה וְאֶחָד לְמַטָּה, שְׁחִיטָתָן כְּשֵׁרָה: