Or HaChaim on Exodus 21:29/אור החיים על שמות כ״א:כ״ט

1 והועד בבעליו, and its owner(s) had been warned, etc. The reason the Torah speaks of "its owners" (pl) is that in the event the ox in question is owned jointly by several people, all are guilty of the death penalty; the situation is not comparable to one where two people combined to administer a lethal blow to another. In the latter case both are not culpable before a human tribunal (compare Baba Kama 26). The reason is that there they are only free from a human tribunal, whereas both are guilty in the eyes of Heaven. In our case there is no death penalty by a human court even if the animal belongs to a single owner. When the Torah speaks about the owner יומת, it refers to death at the hands of Heaven. All owners have to pay the required amount of compensation, however.