If both of them have got it, it is most likely to be something that travels bewteen animals - so I think skin scrapes would be useful, but they may not find any mange mites.
I have a GG goatling that has suddenly, over a period of 4 weeks come out on spots all over, she is scratching herself raw in places... Vet has treated for mange (as in Dectomax twice) but no improvement (skin scrapings were clear). So mange is ruled out (also the other two goats sharing her pen are absolutely clear!) Vet is now thinking along the lines of either a) an auto-immune problem or b) a stress reaction that suddenly caused her immune system to not be able to cope with normally present bacteria (most likely staph aureus) and come out in rash all over. I have treated with an initial dose of steroid and now giving twice weekly Betamox injections. Some improvements, but nothing amazing....
On the basis that all my other goats are blooming healthy we have also ruled out something like Zn deficiency.
What is the anti-fly spray that you are using? they could just be allergic to that? Normally goats are not so troubled by flies as are sheep - they are cleaner animals, as long as they do not have the runs.
I would also think that maybe you could take your goats to the vet rather than have him/her coming out to you, it is a lot cheaper that way? If these are pygmy/pygmy crosses - don't they hop into the boot of your car?