dog asthma?

I have 1 Sheltie who is 12 she has had a cough forever and after a lot of vets it seems she has a kind of dog asthma. We recently adopted a 4 year old Sheltie and he too has the same cough. So I am wondering if this is common in shelties? and is there any advise on how to treat this. I have given Sarah my 12 year old my Asthma puffer which seems to help but I am wondering if there is anything else I can do.