Many culled chinchillas are wholesaled to pet stores which is a really good reason to not breed pet store chins. Chinchillas can breed at 8 weeks but not safely until they are 12mo which is why it is best to separate all males from females immediately upon weaning. Pairing older chinchillas can be tough and some animals are not compatible with others. Breeders have interesting stories to tell about pairing techniques. However, it's never wise to let a male and female out of the cage together, because it can happen quite spontaneously. It is likely that any given female will go into heat and breed again after giving birth, which, is one reason you ought to separate the male when it's close to the due date. It is called breed-back and is hard on the female when it happens and the commotion of mating can lead to the death of the kits if they have no place to hide. The uterus has two sections that operate independently and in theory can even have two pregnancies at the same time which result in two consecutive births. This process is called superfoetation but as far as I know it's never been scientifically documented. There are many breeders who seem to have witnessed it though. So by the same physiological logic breed-back is also possible.