Feline (General)

[Suzee]

Hi, does anyone have any suggestions for this problem...
Most of the time eight year old spayed female tolerates three year old female, both indoor pets. Younger cat becomes very agitated when she sees a neighborhood cat in the yard, usually at night when I have neglected to lower the blinds early enough, and since she can't reach them will viciously attack the older cat. This causes hard feelings and distrust, so the animosity can go on for days.
I tried giving them both Rescue Remedy and also their favorite treats while they were right next to each other and also catnip which made them somewhat mellow.
I sure would like to stop the attacking, though. Our back yard is not fenced in, not that it would make any difference to a cat, right?