Midrash for Chullin 159:50

דרבנן סברי

Whence do you gather this? - For it reads: IF BOTH ANIMALS WERE CONSECRATED [AND WERE SLAUGHTERED] OUTSIDE THE SANCTUARY, HE WHO SLAUGHTERED THE FIRST INCURS THE PENALTY OF KARETH, BOTH ANIMALS ARE INVALID, AND EACH INCURS FORTY STRIPES.

Sifrei Devarim