Midrash sur Le Lévitique 19:5

1) (Vayikra 19:5) "And if you slaughter a sacrifice of peace-offerings to the L–rd, for your will shall you slaughter it. (Vayikra 19:6) On the day that you slaughter it, it shall be eaten, and on the next day.": Let this not be stated, (for it has already been written, viz. [Vayikra 7:16]). If it is not needed for eating, learn it as applying to slaughtering, i.e., even when you slaughter it, your intent should be to eat it for two days.

2) This tells me only of peace-offerings. Whence do I derive the same for all (offerings) that are eaten for two days? From "And if you slaughter a sacrifice."

3) (Vayikra 19:5) "shall you slaughter it": two heads are not slaughtered at the same time.