One of my cats has ear mites!

One of my 2 cats has ear mites. I am treating him with ear drops to kill the mite, but should I treat my other cat as well with the drops even though she's not showing any signs of having ear mites and has clean ears??

I wouldn't.  Ear mites are contagious but they are 'slow' contagious.  If the other cat is not showing signs of mites, I wouldn't bother treating at this point.  Now that you know what they look like, if the other cat does develop them, you'll be on top of the situation.

One of the reasons I like using Revolution for flea control is that it also kills ear mites.  You need to give it twice as often to treat ear mites but given once monthly it seems to do a great job preventing them.  So, of course, does a clean environment.