Reference for Chullin 161:29

לרבי שמעון שניהם ענושים כרת

Now if you are to assume that [according to R'Simeon] the law of 'It and its young' does not apply to consecrated animals, then why [is it stated that] he who slaughtered the second has transgressed a negative command and no more?

Jastrow

Jastrow