Reference for Chullin 63:7

חתך דלעת עמה דברי הכל פסולה נחתכה דלעת עמה דברי הכל כשרה:

implying 'it' but not it and another, referred to the slaughtering of two Red Cows simultaneously; but to slaughter a 'common' animal with it, I might have said, would not render it invalid.

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