If two of these die at the same time, the last ability of each will trigger. If each player controlled one, the nonactive player's ability will resolve first. The Underworld Cerberus controlled by that player will be exiled, and all other creature cards in graveyards (including the Underworld Cerberus controlled by the active player) will be returned to their owners' hands. Then the active player's ability will resolve. That Underworld Cerberus won't be exiled, but any creature cards in graveyards at that time, perhaps because they died in response to this ability, will be returned to their owners' hands.