Building a second-hand book

In your book on Talmudic logic, you presented the equilateral inference as based on Occam’s razor and that “the intuitive preference is for the simpler possibility, since it is better to assume that there is only one cause for a given result, rather than that either of two different causes alone could cause it. This is the accepted explanation for the ‘equilateral’ inference.”
But in referring there to your book Two Carts, you explain that there is an a priori principle here (and not just an estimate as in Ockham’s Razor) that every law must have one cause and that “it seems that the assumption that we must make at the basis of the determination of the illegitimacy of such a division is that every law has only one cause, and nothing else. That is, the Oral Torah, which accepts the teaching of ‘which side’ as legitimate, assumes that a law is not possible that is caused by two different causes (which are not private cases of one general cause), or two different causes.”
My question is, is this really a necessary assumption (and all we have to do is narrow down the scope of the common generalization to two or more of the causes that create the result) or is the assumption that if there is an identical result, we must make an inference of the equal side also not necessary? Beyond that, do you generally believe that a cause is always a sufficient and necessary factor or can it only be sufficient and not necessary?