I am sorry to hear your dogs are so itchy. My name is Rebecca. I am a veterinarian and will do my best to help.
Since both dogs are affected and have not had problem in the past, the first thing I would do is to use a good flea product such as Frontline, or Seresto collars, or Nexgard or Bravecto. I know you are probably not seeing fleas, but that does not mean it is not due to fleas! When dogs become sensitized to flea bites, it only takes 1 or 2 fleas on their body to set them off scratching, and it can be hard to see a single tiny flea on a dog. I like Frontline since it is not absorbed into the blood stream or body, and the Seresto collars are so safe that if a dog eats one, it dissolves in the stomach.
I once saw a bald cat, no hair at all, had been bald for years. When Frontline came out the owners put it on him, and all his hair grew back! It had been fleas all along and you would think you could see fleas on a totally bald cat.
So that is the first thing to try. If it is a food allergy, a grain free diet does not help usually; most food allergies are to a meat protein. It just seems unlikely that both dogs would suddenly develop a food allergy at the same time.
There is a great new prescription itch/allergy drug called Apoquel that stops about 80% of dogs with itch due to allergy, but I would try a good flea product (not an over the counter Hartz Mountain or Sergeant's ) first.
Let me know if this is helpful and what other questions I can answer.