This could be from fleas if you do not have them on flea prevention such as Frontline. Even if you don't see any fleas one bite can make a flea allergic dog itch all over.
Other possibilities include ringworm fungal infection, bacterial infection, food allergies, mange mites, contact dermatitis, use of shampoos or pesticides the dog reacts poorly to etc.
As you have two dogs with the same problem you really should see your vet with them. The vet can do a skin scraping and check for mites and fleas and fungal infections or bacteria etc.
http://www.thepetcenter.com/gen/itch.html
Hope this helps you!
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