Ferrets do lose hair during shedding season but this will normally only be on the tail and will be quickly replaced by new hair. One of the most common cause of hair loss is ferrets is adrenal tumors, which is normally treated by surgery to remove the affected gland, as drugs are only available to treat the symptoms not the disease. The fact that your second ferret has now started to show the same symptoms however indicates that it is more likely to be a result of skin parasites such as mites (or a fungal infection). I would recommend taking them to the vet who can so a skin scrape and a thorough exam to identify any external causative agent. There are simple treatments such as revolution which can be used if mites/fleas are the problem.