Hullin, Chapter Two, Mishnah Two
1) If one slaughtered two animals simultaneously, the slaughtering is valid.
2) If two persons held the knife and slaughtered, even if one cut higher up and the other cut lower down [in the neck], the slaughtering is valid.
Section one: As long as the slaughtering is performed in a valid manner, if two animals are slaughtered together they are both valid.
Section two: Similarly, if two people hold the knife, even if they hold different parts of the knife, the slaughtering is valid, as long as it was performed in a valid manner.