My two dogs keep scratching and biting at their tails and back legs.  Have used two different flea and tick treatments to no avail.  No sign of fleas or flea dirt on any of them.  The one has scrached and biten himself till the fur has come out.  I have just treated them with Advocate and bathing them in Maloseb Shampoo just in case they have picked up a mite.  Does anyone ever had anything like this happen to them?

Check out their anal glands?  That can make them bite their rear halfs.  Mind you - odd that it's both of them - perhaps it's an allergy to something like fertiliser or other spray where you have walked them?  In which case both time & the shampoo will help.