Summary:
Three people who dug ten טפחים to complete a בור of thirty טפחים are all equally liable for the death of an animal that fell into this pit. If one dug ten and the other one more for a total of eleven טפחים, then only the first is חייב. Similarly by מרבה בחבילות, if the second fire could damage on its own then they are both חייב. However if one could not damage; only the other, then the חיוב is only on the one who could damage.