Have you tried adding any extra oils to their diets? A good Omega oil can help. Such as Salmon oil or Flax seed oil, or even Dr. Udo's blend. I know they have some esp for animals, by my holistic vet often suggests using human supplements as the quality is more regulated than the quality of animal supplements. As for amount, you want to start with just a little added to their food (either once or twice) per day, and then build it up little by little until their poos are getting soft...then back off a little until everything is running fine
Good luck...and here's keeping my fingers crossed for an early Spring!