It may not be fleas. My cat has for years gotten a "scabby" buildup on the back of her ears. It happens about once or twice a year. I did get it checked out and the scabs turned out to be pimples!
What I do for her is use a alcohol swab and rub on the scabs. I will also rub her ears till the scabs come off. Kind of gross but she is my baby... I do wait until the scabs are mostly "dried out" prior to rubbing them off. Basically, I use thumb pressure and NOT my nails to remove the scabs. The scabs are there for healing purposes and should only be removed when that process is done.
I would suggest using a spray hydrocortosine and anti-bacterial spray or ointment. You can use neosporin but not too much. I would also suspect that your cats maybe repeatedly going somewhere injuring themselves or biting eachother. If you have collars on your cats they may get them caught under something causing them to scrape their body's to get free. All of this causing the same type of scabbing or sores.
Frontline is good to use for the flea prevention and using some flea killer carpet powder as well. I'd be careful and NOT apply the frontline near the sores as that will hurt.