Maybe if genetics were that cut and dry, just black and white but they're not. Just because you haven't seen it or may be aware of it happening doesn't mean it can't happen. If two males fertilized the same egg, I'll use the example given earlier with a spider male and pin male to a female. Then like identical twins the ovum should split into two separate embryos. With one being a spider and the other a pin or a normal. Not all identical twins are identical. look up Semi Identical twins. Genetics find a way to get the job done one way or another, sometimes things work out and sometimes not so much the right way but it happens whether all embryos live or not.