Is it safe to assume from what you've written (refer my highlights) that you're saying your dogs were both found with having ear infections? (Otherwise, why the antibiotics and cortisone?)
Do you use spot-ons regularly? Is it possible THAT's what is causing irritation?
The vet tried everything after flea, mite teatments, ear inspections (all clean) in order to close all possible doors. We use Frontline and have been doing so for years, ie long before this happened, with no adverse results